From 525ea2028ca23f86e96c79d9e1aff1db2d22daa8 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Sat, 8 Aug 2026 18:39:01 -0300
Subject: [PATCH 01/22] Try a new kind of very specialized parser for small
 types

---
 hpgsql-tests/EncodingDecodingSpec.hs          |  61 ++++++++-
 hpgsql/src/Hpgsql/Encoding.hs                 | 123 ++++++++++++++----
 .../src/Hpgsql/Encoding/BinarySerializer.hs   |  55 ++++++++
 hpgsql/src/Hpgsql/SimpleParser.hs             |  46 +++++++
 4 files changed, 258 insertions(+), 27 deletions(-)

diff --git a/hpgsql-tests/EncodingDecodingSpec.hs b/hpgsql-tests/EncodingDecodingSpec.hs
index 33450cb..ef2eec9 100644
--- a/hpgsql-tests/EncodingDecodingSpec.hs
+++ b/hpgsql-tests/EncodingDecodingSpec.hs
@@ -11,7 +11,7 @@ import Data.CaseInsensitive (CI)
 import qualified Data.CaseInsensitive as CI
 import Data.Functor ((<&>))
 import Data.Functor.Contravariant (contramap)
-import Data.Int (Int16, Int32, Int64)
+import Data.Int (Int16, Int32, Int64, Int8)
 import qualified Data.List as List
 import qualified Data.Map.Strict as Map
 import Data.Maybe (isNothing)
@@ -35,6 +35,7 @@ import DbUtils
     testConnInfo,
     withRollback,
   )
+import Debug.Trace
 import GHC.Float (float2Double)
 import GHC.Generics (Generic)
 import Hedgehog (PropertyT, annotateShow, (===))
@@ -44,8 +45,7 @@ import qualified Hedgehog.Range as Gen
 import Hpgsql
 import Hpgsql.Connection (ConnectOpts (..), connect, connectOpts, defaultConnectOpts, refreshTypeInfoCache, withConnectionOpts)
 import Hpgsql.Encoding (EncodingContext (..), FieldDecoder (..), FieldEncoder (..), FieldInfo (..), FromPgField (..), FromPgRow (..), LowerCasedPgEnum (..), RowEncoder (..), ToPgField (..), ToPgRow (..), compositeTypeDecoder, compositeTypeEncoder, nullableField, rawBytesFieldDecoder, singleField, typeFieldDecoder, typeFieldEncoder, typeMustBeNamed, typeOidWithName)
-import Hpgsql.InternalTypes (DataRow (..))
-import Hpgsql.Pipeline (pipeline, pipelineWith, runPipeline)
+import Hpgsql.Pipeline (pipeline, pipeline1With, pipelineWith, runPipeline)
 import Hpgsql.Query (mkQuery, sql, vALUES)
 import Hpgsql.Time (Unbounded (..))
 import Hpgsql.TypeInfo (Oid, TypeInfo (..), lookupTypeByOid)
@@ -142,6 +142,9 @@ spec = parallel $ do
     it
       "Values type round-trip"
       valuesTypeRoundTrip
+    it
+      "Especially optimized less-than-4-bytes long value decoders work"
+      smallerThan4BytesValuesAndNullsRoundtrip
   aroundConn $ describe "Custom types" $ do
     it "Composite type" queryCompositeType
     it
@@ -178,9 +181,59 @@ zeroColumnsResults = do
 
 valuesRoundTrip :: HPgConnection -> IO ()
 valuesRoundTrip conn = do
-  let row = ((-49) :: Int, False :: Bool, 2 :: Int16, 3 :: Int32, fromGregorian 1900 02 28, 42 :: Int64, UTCTime (fromGregorian 1999 12 31) 0, '意' :: Char, '&' :: Char, CalendarDiffTime 3 86403, Aeson.Null)
+  let row = ((-49) :: Int, False :: Bool, 2 :: Int16, 3 :: Int32, fromGregorian 1900 02 28, 42 :: Int64, UTCTime (fromGregorian 1999 12 31) 0, '意' :: Char, '&' :: Char, CalendarDiffTime 3 86403, Nothing :: Maybe Bool)
   queryWith rowDecoder conn (mkQuery "SELECT $1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11" row) `shouldReturn` [row]
 
+smallerThan4BytesValuesAndNullsRoundtrip :: HPgConnection -> PropertyT IO ()
+smallerThan4BytesValuesAndNullsRoundtrip conn = hedgehog $ do
+  yearForDate :: Integer <- Gen.forAll $ Gen.integral (Gen.linear 1 9999)
+  month :: Int <- Gen.forAll $ Gen.int $ Gen.linear 1 12
+  day :: Int <- Gen.forAll $ Gen.int $ Gen.linear 1 28
+  date <- Gen.forAll $ Gen.element [Just $ fromGregorian yearForDate month day, Nothing]
+  let i16Boundary :: [Int16]
+      i16Boundary =
+        [minBound .. minBound + 10]
+          ++ [maxBound - 10 .. maxBound]
+          ++ [2 ^ (14 :: Int) - 10 .. 2 ^ (14 :: Int) + 10]
+          ++ [-(2 ^ (14 :: Int)) - 10 .. -(2 ^ (14 :: Int)) + 10]
+      i32Boundary :: [Int32]
+      i32Boundary =
+        [minBound .. minBound + 10]
+          ++ [maxBound - 10 .. maxBound]
+          ++ [2 ^ (30 :: Int) - 10 .. 2 ^ (30 :: Int) + 10]
+          ++ [-(2 ^ (30 :: Int)) - 10 .. -(2 ^ (30 :: Int)) + 10]
+  i16 :: Maybe Int16 <- Gen.forAll $ Gen.choice [Just <$> Gen.element i16Boundary, Just <$> Gen.integral (Gen.linear (-10) 10), pure Nothing]
+  i32 :: Maybe Int32 <- Gen.forAll $ Gen.choice [Just <$> Gen.element i32Boundary, Just <$> Gen.integral (Gen.linear (-10) 10), pure Nothing]
+  b :: Maybe Bool <- Gen.forAll $ Gen.choice [Just <$> Gen.bool, pure Nothing]
+  -- TODO: float4, char
+  -- TODO: Varying recvChunkSize sizes for this test
+  -- TODO: More variations of rows
+  -- TODO: Test `singleField fieldDecoder` as well: we now have two implementations to test for each
+  --       of these types.
+  -- TODO: test errors when trying to decode NULL::type into a non-Maybe in Haskell
+  let r1 = (date, i16, i32, b)
+      r2 = (i16, date, i32, b)
+      r3 = (i32, date, i16, b)
+      r4 = (b, date, i16, i32)
+      r5 = (b, i32, i16, date)
+      r6 = (b, date, i32, i16)
+  (resR1, resR2, resR3, resR4, resR5, resR6) <-
+    liftIO $
+      runPipeline conn $
+        (,,,,,)
+          <$> pipeline1With rowDecoder [sql|SELECT * FROM (^{vALUES [r1]}) subq|]
+          <*> pipeline1With rowDecoder [sql|SELECT * FROM (^{vALUES [r2]}) subq|]
+          <*> pipeline1With rowDecoder [sql|SELECT * FROM (^{vALUES [r3]}) subq|]
+          <*> pipeline1With rowDecoder [sql|SELECT * FROM (^{vALUES [r4]}) subq|]
+          <*> pipeline1With rowDecoder [sql|SELECT * FROM (^{vALUES [r5]}) subq|]
+          <*> pipeline1With rowDecoder [sql|SELECT * FROM (^{vALUES [r6]}) subq|]
+  liftIO resR1 >>= (=== r1)
+  liftIO resR2 >>= (=== r2)
+  liftIO resR3 >>= (=== r3)
+  liftIO resR4 >>= (=== r4)
+  liftIO resR5 >>= (=== r5)
+  liftIO resR6 >>= (=== r6)
+
 byteaValuesRoundTrip :: HPgConnection -> PropertyT IO ()
 byteaValuesRoundTrip conn = hedgehog $ do
   let genBs = Gen.bytes (Gen.linear 0 50)
diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index dd5378f..02d1d6a 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -181,8 +181,17 @@ singleField (FieldDecoder {..}) =
     }
 
 class FromPgField a where
+  -- | A decoder that takes
   fieldDecoder :: FieldDecoder a
 
+  -- | This should be semantically equivalent to `singleField fieldDecoder`, and
+  -- it is automatically derived to be exactly that.
+  -- So as a user, you don't need to override this.
+  -- This field exists for a performance optimization within hpgsql, or for users
+  -- that really know what they're doing.
+  singleFieldRowDecoder :: RowDecoder a
+  singleFieldRowDecoder = singleField fieldDecoder
+
 class FromPgRow a where
   rowDecoder :: RowDecoder a
   default rowDecoder :: (Generic a, ProductTypeDecoder (Rep a)) => RowDecoder a
@@ -254,43 +263,43 @@ compositeTypeEncoder rowEnc =
     }
 
 instance (FromPgField a) => FromPgRow (Only a) where
-  rowDecoder = Only <$> singleField fieldDecoder
+  rowDecoder = Only <$> singleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b) => FromPgRow (a, b) where
-  rowDecoder = (,) <$> singleField fieldDecoder <*> singleField fieldDecoder
+  rowDecoder = (,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c) => FromPgRow (a, b, c) where
-  rowDecoder = (,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder
+  rowDecoder = (,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d) => FromPgRow (a, b, c, d) where
-  rowDecoder = (,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder
+  rowDecoder = (,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e) => FromPgRow (a, b, c, d, e) where
-  rowDecoder = (,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder
+  rowDecoder = (,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f) => FromPgRow (a, b, c, d, e, f) where
-  rowDecoder = (,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder
+  rowDecoder = (,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g) => FromPgRow (a, b, c, d, e, f, g) where
-  rowDecoder = (,,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder
+  rowDecoder = (,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g, FromPgField h) => FromPgRow (a, b, c, d, e, f, g, h) where
-  rowDecoder = (,,,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder
+  rowDecoder = (,,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g, FromPgField h, FromPgField i) => FromPgRow (a, b, c, d, e, f, g, h, i) where
-  rowDecoder = (,,,,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder
+  rowDecoder = (,,,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g, FromPgField h, FromPgField i, FromPgField j) => FromPgRow (a, b, c, d, e, f, g, h, i, j) where
-  rowDecoder = (,,,,,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder
+  rowDecoder = (,,,,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g, FromPgField h, FromPgField i, FromPgField j, FromPgField k) => FromPgRow (a, b, c, d, e, f, g, h, i, j, k) where
-  rowDecoder = (,,,,,,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder
+  rowDecoder = (,,,,,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g, FromPgField h, FromPgField i, FromPgField j, FromPgField k, FromPgField l) => FromPgRow (a, b, c, d, e, f, g, h, i, j, k, l) where
-  rowDecoder = (,,,,,,,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder
+  rowDecoder = (,,,,,,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g, FromPgField h, FromPgField i, FromPgField j, FromPgField k, FromPgField l, FromPgField m) => FromPgRow (a, b, c, d, e, f, g, h, i, j, k, l, m) where
-  rowDecoder = (,,,,,,,,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder
+  rowDecoder = (,,,,,,,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
 
 data FieldEncoder a = FieldEncoder
   { toTypeOid :: !(EncodingContext -> Maybe Oid),
@@ -741,6 +750,19 @@ binaryIntDecoder typOid = \bs ->
       | otherwise = error "Bug in Hpgsql. Decoding binary integral type not an int2, int4 or int8"
     doesFit = maxBoundPgType <= fromIntegral (maxBound @a)
 
+-- | Specialized/performance-oriented Big-Endian binary decoder for Haskell's various IntXX types.
+binaryIntSpecializedRowDecoder :: Parser.Parser (Maybe Int)
+binaryIntSpecializedRowDecoder = do
+  fieldLen <- Parser.takeInt32BE
+  -- TODO: We're assuming `Int` is always 64 bits, so 64bit CPUs? Is that ok?
+  -- TODO: Is there a way to optimistically assume <=4 bytes and use our custom new parser?
+  case fieldLen of
+    4 -> Just . fromIntegral <$> Parser.takeInt32BE
+    (-1) -> pure Nothing
+    8 -> Just . fromIntegral <$> Parser.takeInt64BE
+    2 -> Just . fromIntegral <$> Parser.takeInt16BE
+    _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
+
 binaryFloat4Decoder :: ByteString -> Float
 binaryFloat4Decoder = castWord32ToFloat . either error id . BinSer.decodeWord32BE
 
@@ -774,6 +796,37 @@ instance FromPgField Int where
                 Nothing -> Left "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`",
         allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
       }
+  singleFieldRowDecoder =
+    let fromNullable = \case
+          Nothing -> fail "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`"
+          Just i -> pure i
+     in RowDecoder
+          { fullRowDecoder = const $ binaryIntSpecializedRowDecoder >>= fromNullable,
+            rowColumnsTypeCheck = \case
+              [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` haskellIntOids)]
+              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+            numExpectedColumns = 1
+          }
+
+-- instance {-# OVERLAPPING #-} FromPgField (Maybe Int) where
+--   fieldDecoder = error "NOOO"
+
+--   -- FieldDecoder
+--   --   { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
+--   --       let !decode = binaryIntDecoder oid
+--   --        in \case
+--   --             Just bs -> Just <$> decode bs
+--   --             Nothing -> Right Nothing,
+--   --     allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
+--   --   }
+--   singleFieldRowDecoder =
+--     RowDecoder
+--       { fullRowDecoder = const binaryIntSpecializedRowDecoder,
+--         rowColumnsTypeCheck = \case
+--           [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` haskellIntOids)]
+--           _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+--         numExpectedColumns = 1
+--       }
 
 instance FromPgField Int16 where
   fieldDecoder =
@@ -922,6 +975,18 @@ instance FromPgField Bool where
   fieldDecoder = parsePgType [boolOid] $ \case
     Just bs -> Right $ bs == binaryTrue
     Nothing -> Left "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
+  singleFieldRowDecoder =
+    let dec = Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1
+        word8ToBool = \case
+          Nothing -> fail "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
+          Just w8 -> pure $ w8 == 1
+     in RowDecoder
+          { fullRowDecoder = const $ dec >>= word8ToBool,
+            rowColumnsTypeCheck = \case
+              [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid == boolOid)]
+              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+            numExpectedColumns = 1
+          }
 
 instance FromPgField Char where
   fieldDecoder =
@@ -1066,6 +1131,18 @@ instance FromPgField Day where
       jd <- BinSer.decodeInt32BE 0 bs
       Right $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
     Nothing -> Left "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
+  singleFieldRowDecoder =
+    let dec = Parser.takeInt32BEWithFieldLength
+        int32ToDay = \case
+          Nothing -> fail "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
+          Just i32 -> let jd = fromIntegral i32 :: Integer in pure $ addJulianDurationClip (CalendarDiffDays 0 (jd - 13)) $ fromJulian 2000 01 01
+     in RowDecoder
+          { fullRowDecoder = const $ dec >>= int32ToDay,
+            rowColumnsTypeCheck = \case
+              [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid == dateOid)]
+              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+            numExpectedColumns = 1
+          }
 
 instance FromPgField (Unbounded Day) where
   fieldDecoder = parsePgType [dateOid] $ \case
@@ -1105,15 +1182,13 @@ instance FromPgField Aeson.Value where
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let
-              -- jsonb has a byte prepended to the contents and json does not
-              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in
-              \case
-                Just bs -> case Aeson.decodeStrict $ fixJsonb bs of
-                  Just d -> Right d
-                  Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid."
-                Nothing -> Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
+            let -- jsonb has a byte prepended to the contents and json does not
+                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in \case
+                  Just bs -> case Aeson.decodeStrict $ fixJsonb bs of
+                    Just d -> Right d
+                    Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid."
+                  Nothing -> Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
 
@@ -1151,7 +1226,7 @@ instance {-# OVERLAPPING #-} forall a. (FromPgField a) => FromPgField (Vector (V
       { fieldValueDecoder = \colInfo ->
           let !arrayFieldDecoder = arrayParser colInfo.encodingContext <* Parser.endOfInput
            in \case
-                Nothing -> Left "Cannot decode SQL null as the Haskell Vector type. Use a `Maybe (Vector (Vector a))`"
+                Nothing -> Left "Cannot decode SQL null as the Haskell (Vector (Vector a)) type. Use a `Maybe (Vector (Vector a))`"
                 Just bs -> case Parser.parseOnly arrayFieldDecoder bs of
                   Parser.ParseOk v -> Right v
                   Parser.ParseFail err -> Left err,
@@ -1185,6 +1260,8 @@ instance {-# OVERLAPPING #-} forall a. (FromPgField a) => FromPgField (Vector (V
                 Left err -> fail $ "Error parsing array element: " ++ show err
                 Right el -> pure el
 
+{-# INLINE genericFromPgRow #-}
+
 -- | Derives `FromPgRow` generically.
 genericFromPgRow :: forall a. (Generic a, ProductTypeDecoder (Rep a)) => RowDecoder a
 genericFromPgRow = to <$> genRowDecoder @(Rep a)
diff --git a/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs b/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
index cfaa2c8..8f714f1 100644
--- a/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
+++ b/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
@@ -22,10 +22,13 @@ module Hpgsql.Encoding.BinarySerializer
     encodeInt16BE,
     encodePgBoolean,
     decodeDataRow,
+    decodePgFieldWithAtMost4Bytes,
+    WordDecoding (..),
   )
 where
 
 import Data.ByteString (ByteString)
+import qualified Data.ByteString as BS
 import qualified Data.ByteString.Internal as InternalBS
 import Data.Int (Int16, Int32, Int64)
 import Prelude hiding (encodeFloat)
@@ -130,6 +133,8 @@ encodeFloat n = unsafeEncodeWord (castFloatToWord32 n) fromBigEndian32 4
 encodeDouble :: Double -> ByteString
 encodeDouble n = unsafeEncodeWord (castDoubleToWord64 n) fromBigEndian64 8
 
+-- TODO: Encode field length together with value for small types.
+-- This can also be a performance boost by having fewer bytestrings?
 {-# INLINE encodePgBoolean #-}
 encodePgBoolean :: Bool -> ByteString
 encodePgBoolean v = if v then "\SOH" else "\NUL"
@@ -173,3 +178,53 @@ decodeDataRow idx bs@(InternalBS.BS _bytesPtr len) =
     toResult lenFullMsg
       | len >= 1 + lenFullMsg + idx.idx = Right $ ByteStringIdx $ 1 + lenFullMsg + idx.idx
       | otherwise = Left "Less than enough bytes to decode a full DataRow"
+
+{-# INLINE decodePgFieldWithAtMost4Bytes #-}
+
+data WordDecoding a where
+  TypeSize1 :: WordDecoding Word8
+  TypeSize2 :: WordDecoding Word16
+  TypeSize4 :: WordDecoding Word32
+
+-- | A specialized decoder that decoders a query result's
+-- field's contents, but only for PG fields at most 4 bytes long and
+-- at least 1 byte long (so no text or void types, for example).
+-- This includes essentially int32, int16, and booleans.
+-- Pass in as type argument a Word8, Word16 or Word32 to indicate
+-- the size of the PG type you're decoding.
+decodePgFieldWithAtMost4Bytes :: forall a. (Storable a, Integral a) => WordDecoding a -> ByteString -> Either String (Maybe a, ByteString)
+decodePgFieldWithAtMost4Bytes wdec =
+  let (pgTypeSize, endianSwap, valueMask :: Word64) = case wdec of
+        TypeSize1 -> (1, Prelude.id, 0b00000000_00000000_00000000_00000000_11111111_00000000_00000000_00000000)
+        TypeSize2 -> (2, fromBigEndian16, 0b00000000_00000000_00000000_00000000_11111111_11111111_00000000_00000000)
+        TypeSize4 -> (4, fromBigEndian32, 0b00000000_00000000_00000000_00000000_11111111_11111111_11111111_11111111)
+      valueShift :: Int = 8 * (4 - pgTypeSize)
+   in \bs ->
+        -- We try the most optimistic case first:
+        -- - Non-null 4 byte long types (like int32)
+        -- - Null int32 followed by at least one other field (not the last field in the row)
+        -- - Shorter types (int16, bool) followed by at least one other field (not the last field in the row)
+        -- In all the cases above, there are at least 8 bytes in the row, so our decoding into a Word64 will succeed.
+        case unsafeDecodeWord bs 8 fromBigEndian64 of
+          Right (w64 :: Word64) ->
+            let fieldLenW64 :: Word64 = flip unsafeShiftR 32 $ w64 .&. 0b11111111_11111111_11111111_11111111_00000000_00000000_00000000_00000000
+                fieldIfNotNull :: a = fromIntegral $ unsafeShiftR (w64 .&. valueMask) valueShift
+             in if fieldLenW64 == 0xFFFFFFFF -- (-1) in two's-complement
+                  then
+                    Right (Nothing, BS.drop 4 bs)
+                  else
+                    if fieldLenW64 <= 4 -- TODO: Maybe we omit this check and make this an unsafe function?
+                      then
+                        Right $ (Just fieldIfNotNull, BS.drop (4 + fromIntegral fieldLenW64) bs) -- This avoids another load instruction
+                      else Left "You cannot use decodePgFieldWithAtMost4Bytes to decode fields of types potentially more than 4 bytes long"
+          Left _ -> do
+            -- This is the not-as-optimistic case, which includes:
+            -- - A NULL int32 as the last field in the row
+            -- - A bool/int8/int16 that is the last field in the row
+            lenField <- decodeInt32BE 0 bs
+            if lenField >= 0
+              then do
+                -- peek after the next 4 bytes for @a
+                fieldValue <- unsafeDecodeWordOffset 4 bs (fromIntegral pgTypeSize) endianSwap
+                Right (Just fieldValue, BS.drop (4 + fromIntegral lenField) bs)
+              else Right (Nothing, BS.drop 4 bs) -- TODO: Return "" as an empty bytestring
diff --git a/hpgsql/src/Hpgsql/SimpleParser.hs b/hpgsql/src/Hpgsql/SimpleParser.hs
index abac6d4..a9a6203 100644
--- a/hpgsql/src/Hpgsql/SimpleParser.hs
+++ b/hpgsql/src/Hpgsql/SimpleParser.hs
@@ -24,6 +24,10 @@ module Hpgsql.SimpleParser
     takeDataRow,
     parseManyRows,
     skip,
+    parsePgFieldWithAtMost4Bytes,
+    takeInt64BEWithFieldLength,
+    takeInt32BEWithFieldLength,
+    takeInt16BEWithFieldLength,
   )
 where
 
@@ -31,6 +35,7 @@ import Data.ByteString (ByteString)
 import qualified Data.ByteString as BS
 import Data.Int (Int16, Int32, Int64)
 import Hpgsql.Encoding.BinarySerializer (ByteStringIdx (..))
+import Foreign.Storable (Storable)
 import qualified Hpgsql.Encoding.BinarySerializer as BinSer
 import Prelude hiding (take)
 
@@ -119,6 +124,15 @@ takeInt16BE = Parser $ \idx bs kf ks ->
     Left err -> kf err
     Right v -> ks v (idx + 2) bs
 
+{-# INLINE takeInt16BEWithFieldLength #-}
+
+-- | Parses both a field length and the field itself, for
+-- an Int16 in a row.
+takeInt16BEWithFieldLength :: Parser (Maybe Int16)
+takeInt16BEWithFieldLength = do
+  mi16 <- parsePgFieldWithAtMost4Bytes BinSer.TypeSize2
+  pure $ fromIntegral <$> mi16
+
 {-# INLINE takeInt32BE #-}
 takeInt32BE :: Parser Int32
 takeInt32BE = Parser $ \idx bs kf ks ->
@@ -126,6 +140,26 @@ takeInt32BE = Parser $ \idx bs kf ks ->
     Left err -> kf err
     Right v -> ks v (idx + 4) bs
 
+{-# INLINE takeInt32BEWithFieldLength #-}
+
+-- | Parses both a field length and the field itself, for
+-- an Int32 in a row.
+takeInt32BEWithFieldLength :: Parser (Maybe Int32)
+takeInt32BEWithFieldLength = do
+  mi32 <- parsePgFieldWithAtMost4Bytes BinSer.TypeSize4
+  pure $ fromIntegral <$> mi32
+
+{-# INLINE takeInt64BEWithFieldLength #-}
+
+-- | Parses both a field length and the field itself, for
+-- an Int64 in a row.
+takeInt64BEWithFieldLength :: Parser (Maybe Int64)
+takeInt64BEWithFieldLength = do
+  fieldLen <- takeInt32BE
+  if fieldLen == (-1)
+    then pure Nothing
+    else Just <$> takeInt64BE
+
 {-# INLINE takeInt64BE #-}
 takeInt64BE :: Parser Int64
 takeInt64BE = Parser $ \idx bs kf ks ->
@@ -143,6 +177,18 @@ takeDataRow = Parser $ \idx bs kf ks ->
     Left err -> kf err
     Right idxRest -> ks idxRest idxRest bs
 
+{-# INLINE parsePgFieldWithAtMost4Bytes #-}
+
+-- | A specialized parser that reads a query result's
+-- field's contents.
+parsePgFieldWithAtMost4Bytes :: forall a. (Storable a, Integral a) => BinSer.WordDecoding a -> Parser (Maybe a)
+parsePgFieldWithAtMost4Bytes wdec =
+  let dec = BinSer.decodePgFieldWithAtMost4Bytes wdec
+   in Parser $ \bs kf ks ->
+        case dec bs of
+          Left err -> kf err
+          Right (v, rest) -> ks v rest
+
 parseMany :: Parser a -> Parser [a]
 parseMany p = Parser $ \idx' bs' _kf ks -> let (vs, restIdx) = go idx' bs' in ks vs restIdx bs'
   where

From b4d11149896e40ae271af1ae127f2005b000e9f0 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Wed, 12 Aug 2026 15:01:42 -0300
Subject: [PATCH 02/22] Post-rebase fixing

---
 hpgsql-tests/EncodingDecodingSpec.hs          |  1 +
 hpgsql/src/Hpgsql/Encoding.hs                 | 18 ++++++++-------
 .../src/Hpgsql/Encoding/BinarySerializer.hs   | 22 +++++++++----------
 hpgsql/src/Hpgsql/SimpleParser.hs             |  8 +++----
 4 files changed, 26 insertions(+), 23 deletions(-)

diff --git a/hpgsql-tests/EncodingDecodingSpec.hs b/hpgsql-tests/EncodingDecodingSpec.hs
index ef2eec9..efdbdfc 100644
--- a/hpgsql-tests/EncodingDecodingSpec.hs
+++ b/hpgsql-tests/EncodingDecodingSpec.hs
@@ -45,6 +45,7 @@ import qualified Hedgehog.Range as Gen
 import Hpgsql
 import Hpgsql.Connection (ConnectOpts (..), connect, connectOpts, defaultConnectOpts, refreshTypeInfoCache, withConnectionOpts)
 import Hpgsql.Encoding (EncodingContext (..), FieldDecoder (..), FieldEncoder (..), FieldInfo (..), FromPgField (..), FromPgRow (..), LowerCasedPgEnum (..), RowEncoder (..), ToPgField (..), ToPgRow (..), compositeTypeDecoder, compositeTypeEncoder, nullableField, rawBytesFieldDecoder, singleField, typeFieldDecoder, typeFieldEncoder, typeMustBeNamed, typeOidWithName)
+import Hpgsql.InternalTypes (DataRow (..))
 import Hpgsql.Pipeline (pipeline, pipeline1With, pipelineWith, runPipeline)
 import Hpgsql.Query (mkQuery, sql, vALUES)
 import Hpgsql.Time (Unbounded (..))
diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index 02d1d6a..6df4a8a 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -1182,13 +1182,15 @@ instance FromPgField Aeson.Value where
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let -- jsonb has a byte prepended to the contents and json does not
-                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in \case
-                  Just bs -> case Aeson.decodeStrict $ fixJsonb bs of
-                    Just d -> Right d
-                    Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid."
-                  Nothing -> Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
+            let
+              -- jsonb has a byte prepended to the contents and json does not
+              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in
+              \case
+                Just bs -> case Aeson.decodeStrict $ fixJsonb bs of
+                  Just d -> Right d
+                  Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid."
+                Nothing -> Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
 
@@ -1282,7 +1284,7 @@ instance (FromPgField a) => ProductTypeDecoder (K1 r a) where
   -- coercing instead of fmap reduces memory usage, apparently
   -- by reducing (unnecessary) closures in the final row decoder,
   -- as per looking at GHC Core
-  genRowDecoder = coerce $ singleField $ fieldDecoder @a
+  genRowDecoder = coerce $ singleFieldRowDecoder @a
 
 genericToPgRow :: forall a. (Generic a, ProductTypeEncoder (Rep a)) => RowEncoder a
 genericToPgRow = contramap from genRowEncoder
diff --git a/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs b/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
index 8f714f1..fabab6e 100644
--- a/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
+++ b/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
@@ -28,7 +28,6 @@ module Hpgsql.Encoding.BinarySerializer
 where
 
 import Data.ByteString (ByteString)
-import qualified Data.ByteString as BS
 import qualified Data.ByteString.Internal as InternalBS
 import Data.Int (Int16, Int32, Int64)
 import Prelude hiding (encodeFloat)
@@ -192,39 +191,40 @@ data WordDecoding a where
 -- This includes essentially int32, int16, and booleans.
 -- Pass in as type argument a Word8, Word16 or Word32 to indicate
 -- the size of the PG type you're decoding.
-decodePgFieldWithAtMost4Bytes :: forall a. (Storable a, Integral a) => WordDecoding a -> ByteString -> Either String (Maybe a, ByteString)
+-- Returns the index into the first yet-unparsed byte.
+decodePgFieldWithAtMost4Bytes :: forall a. (Storable a, Integral a) => WordDecoding a -> ByteStringIdx -> ByteString -> Either String (Maybe a, ByteStringIdx)
 decodePgFieldWithAtMost4Bytes wdec =
   let (pgTypeSize, endianSwap, valueMask :: Word64) = case wdec of
         TypeSize1 -> (1, Prelude.id, 0b00000000_00000000_00000000_00000000_11111111_00000000_00000000_00000000)
         TypeSize2 -> (2, fromBigEndian16, 0b00000000_00000000_00000000_00000000_11111111_11111111_00000000_00000000)
         TypeSize4 -> (4, fromBigEndian32, 0b00000000_00000000_00000000_00000000_11111111_11111111_11111111_11111111)
       valueShift :: Int = 8 * (4 - pgTypeSize)
-   in \bs ->
+   in \idx bs ->
         -- We try the most optimistic case first:
         -- - Non-null 4 byte long types (like int32)
         -- - Null int32 followed by at least one other field (not the last field in the row)
         -- - Shorter types (int16, bool) followed by at least one other field (not the last field in the row)
         -- In all the cases above, there are at least 8 bytes in the row, so our decoding into a Word64 will succeed.
-        case unsafeDecodeWord bs 8 fromBigEndian64 of
+        case unsafeDecodeWord idx bs 8 fromBigEndian64 of
           Right (w64 :: Word64) ->
             let fieldLenW64 :: Word64 = flip unsafeShiftR 32 $ w64 .&. 0b11111111_11111111_11111111_11111111_00000000_00000000_00000000_00000000
                 fieldIfNotNull :: a = fromIntegral $ unsafeShiftR (w64 .&. valueMask) valueShift
              in if fieldLenW64 == 0xFFFFFFFF -- (-1) in two's-complement
                   then
-                    Right (Nothing, BS.drop 4 bs)
+                    Right (Nothing, idx + 4)
                   else
-                    if fieldLenW64 <= 4 -- TODO: Maybe we omit this check and make this an unsafe function?
+                    if fieldLenW64 <= 4
                       then
-                        Right $ (Just fieldIfNotNull, BS.drop (4 + fromIntegral fieldLenW64) bs) -- This avoids another load instruction
+                        Right (Just fieldIfNotNull, idx + 4 + fromIntegral fieldLenW64)
                       else Left "You cannot use decodePgFieldWithAtMost4Bytes to decode fields of types potentially more than 4 bytes long"
           Left _ -> do
             -- This is the not-as-optimistic case, which includes:
             -- - A NULL int32 as the last field in the row
             -- - A bool/int8/int16 that is the last field in the row
-            lenField <- decodeInt32BE 0 bs
+            lenField <- decodeInt32BE idx bs
             if lenField >= 0
               then do
                 -- peek after the next 4 bytes for @a
-                fieldValue <- unsafeDecodeWordOffset 4 bs (fromIntegral pgTypeSize) endianSwap
-                Right (Just fieldValue, BS.drop (4 + fromIntegral lenField) bs)
-              else Right (Nothing, BS.drop 4 bs) -- TODO: Return "" as an empty bytestring
+                fieldValue <- unsafeDecodeWord (idx + 4) bs (fromIntegral pgTypeSize) endianSwap
+                Right (Just fieldValue, idx + 4 + fromIntegral lenField)
+              else Right (Nothing, idx + 4)
diff --git a/hpgsql/src/Hpgsql/SimpleParser.hs b/hpgsql/src/Hpgsql/SimpleParser.hs
index a9a6203..898177d 100644
--- a/hpgsql/src/Hpgsql/SimpleParser.hs
+++ b/hpgsql/src/Hpgsql/SimpleParser.hs
@@ -34,8 +34,8 @@ where
 import Data.ByteString (ByteString)
 import qualified Data.ByteString as BS
 import Data.Int (Int16, Int32, Int64)
-import Hpgsql.Encoding.BinarySerializer (ByteStringIdx (..))
 import Foreign.Storable (Storable)
+import Hpgsql.Encoding.BinarySerializer (ByteStringIdx (..))
 import qualified Hpgsql.Encoding.BinarySerializer as BinSer
 import Prelude hiding (take)
 
@@ -184,10 +184,10 @@ takeDataRow = Parser $ \idx bs kf ks ->
 parsePgFieldWithAtMost4Bytes :: forall a. (Storable a, Integral a) => BinSer.WordDecoding a -> Parser (Maybe a)
 parsePgFieldWithAtMost4Bytes wdec =
   let dec = BinSer.decodePgFieldWithAtMost4Bytes wdec
-   in Parser $ \bs kf ks ->
-        case dec bs of
+   in Parser $ \idx bs kf ks ->
+        case dec idx bs of
           Left err -> kf err
-          Right (v, rest) -> ks v rest
+          Right (v, restIdx) -> ks v restIdx bs
 
 parseMany :: Parser a -> Parser [a]
 parseMany p = Parser $ \idx' bs' _kf ks -> let (vs, restIdx) = go idx' bs' in ks vs restIdx bs'

From 05e3a9d77e92fa2f84a30420b35951967fb27beb Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Wed, 12 Aug 2026 16:41:25 -0300
Subject: [PATCH 03/22] More specialized instances, more confirmation of
 benefits

---
 hpgsql/src/Hpgsql/Encoding.hs                 | 46 +++++++++++++------
 .../src/Hpgsql/Encoding/BinarySerializer.hs   |  8 ++--
 hpgsql/src/Hpgsql/SimpleParser.hs             | 25 ++++++++--
 3 files changed, 57 insertions(+), 22 deletions(-)

diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index 6df4a8a..b092942 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -157,6 +157,7 @@ instance Applicative RowDecoder where
 instance (TypeError (TypeLits.Text "RowDecoder does not have a Monad instance in Hpgsql because Hpgsql type-checks the result types of queries before having access to even the first data row. Use the Applicative class to write your instances or use the Monadic decoding variants.")) => Monad RowDecoder where
   (>>=) = error "inaccessible bind in Monad RowDecoder instance"
 
+{-# INLINE singleField #-} -- 1.2% wall time perf. gain with this
 singleField :: FieldDecoder a -> RowDecoder a
 singleField (FieldDecoder {..}) =
   RowDecoder
@@ -764,10 +765,10 @@ binaryIntSpecializedRowDecoder = do
     _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
 
 binaryFloat4Decoder :: ByteString -> Float
-binaryFloat4Decoder = castWord32ToFloat . either error id . BinSer.decodeWord32BE
+binaryFloat4Decoder = castWord32ToFloat . either error id . BinSer.decodeWord32BE 0
 
 binaryFloat8Decoder :: ByteString -> Double
-binaryFloat8Decoder = castWord64ToDouble . either error id . BinSer.decodeWord64BE
+binaryFloat8Decoder = castWord64ToDouble . either error id . BinSer.decodeWord64BE 0
 
 parsePgType :: [Oid] -> (Maybe ByteString -> Either String a) -> FieldDecoder a
 parsePgType !requiredTypeOids !fieldValueDecoder =
@@ -808,6 +809,8 @@ instance FromPgField Int where
             numExpectedColumns = 1
           }
 
+-- The instance below makes our Records benchmark faster and use less
+-- memory, but makes our Tuples benchmark slower. Worth investigating.
 -- instance {-# OVERLAPPING #-} FromPgField (Maybe Int) where
 --   fieldDecoder = error "NOOO"
 
@@ -831,9 +834,9 @@ instance FromPgField Int where
 instance FromPgField Int16 where
   fieldDecoder =
     FieldDecoder
-      { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
-          let !decode = binaryIntDecoder oid
-           in \case
+      { fieldValueDecoder =
+          let !decode = binaryIntDecoder int2Oid
+           in const $ \case
                 Just bs -> decode bs
                 Nothing -> Left "Cannot decode SQL null as the Haskell Int16 type. Use a `Maybe Int16`",
         allowedPgTypes = (== int2Oid) . fieldTypeOid
@@ -905,6 +908,23 @@ instance FromPgField Double where
                 Nothing -> Left "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`",
         allowedPgTypes = (`elem` [float8Oid, float4Oid]) . fieldTypeOid
       }
+  singleFieldRowDecoder =
+    let fromNullable = \case
+          Nothing -> fail "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`"
+          Just i -> pure i
+        float4OrDouble8Decoder = do
+          len <- Parser.takeInt32BE
+          case len of
+            8 -> Just <$> Parser.takeDoubleBE
+            4 -> Just . float2Double <$> Parser.takeFloatBE
+            _ -> pure Nothing
+     in RowDecoder
+          { fullRowDecoder = const $ float4OrDouble8Decoder >>= fromNullable,
+            rowColumnsTypeCheck = \case
+              [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` [float8Oid, float4Oid])]
+              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+            numExpectedColumns = 1
+          }
 
 -- | Allows you to specify a type (and other checks, possibly) for a `FieldDecoder`.
 -- This can be useful to ensure you're not accidentally decoding a different type.
@@ -1182,15 +1202,13 @@ instance FromPgField Aeson.Value where
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let
-              -- jsonb has a byte prepended to the contents and json does not
-              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in
-              \case
-                Just bs -> case Aeson.decodeStrict $ fixJsonb bs of
-                  Just d -> Right d
-                  Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid."
-                Nothing -> Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
+            let -- jsonb has a byte prepended to the contents and json does not
+                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in \case
+                  Just bs -> case Aeson.decodeStrict $ fixJsonb bs of
+                    Just d -> Right d
+                    Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid."
+                  Nothing -> Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
 
diff --git a/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs b/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
index fabab6e..4aab61a 100644
--- a/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
+++ b/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
@@ -101,12 +101,12 @@ decodeWord8 :: ByteStringIdx -> ByteString -> Either String Word8
 decodeWord8 idx bs = decodeWord CWord8 idx bs Prelude.id
 
 {-# INLINE decodeWord32BE #-}
-decodeWord32BE :: ByteString -> Either String Word32
-decodeWord32BE bs = decodeWord CWord32 0 bs fromBigEndian32
+decodeWord32BE :: ByteStringIdx -> ByteString -> Either String Word32
+decodeWord32BE idx bs = unsafeDecodeWord idx bs 4 fromBigEndian32
 
 {-# INLINE decodeWord64BE #-}
-decodeWord64BE :: ByteString -> Either String Word64
-decodeWord64BE bs = decodeWord CWord64 0 bs fromBigEndian64
+decodeWord64BE :: ByteStringIdx -> ByteString -> Either String Word64
+decodeWord64BE idx bs = unsafeDecodeWord idx bs 8 fromBigEndian64
 
 {-# INLINE decodeInt32BE #-}
 decodeInt32BE :: ByteStringIdx -> ByteString -> Either String Int32
diff --git a/hpgsql/src/Hpgsql/SimpleParser.hs b/hpgsql/src/Hpgsql/SimpleParser.hs
index 898177d..659f9e1 100644
--- a/hpgsql/src/Hpgsql/SimpleParser.hs
+++ b/hpgsql/src/Hpgsql/SimpleParser.hs
@@ -28,6 +28,8 @@ module Hpgsql.SimpleParser
     takeInt64BEWithFieldLength,
     takeInt32BEWithFieldLength,
     takeInt16BEWithFieldLength,
+    takeFloatBE,
+    takeDoubleBE,
   )
 where
 
@@ -35,6 +37,7 @@ import Data.ByteString (ByteString)
 import qualified Data.ByteString as BS
 import Data.Int (Int16, Int32, Int64)
 import Foreign.Storable (Storable)
+import GHC.Float (castWord32ToFloat, castWord64ToDouble)
 import Hpgsql.Encoding.BinarySerializer (ByteStringIdx (..))
 import qualified Hpgsql.Encoding.BinarySerializer as BinSer
 import Prelude hiding (take)
@@ -121,8 +124,8 @@ skip n = Parser $ \idx bs _ ks ->
 takeInt16BE :: Parser Int16
 takeInt16BE = Parser $ \idx bs kf ks ->
   case BinSer.decodeInt16BE idx bs of
-    Left err -> kf err
     Right v -> ks v (idx + 2) bs
+    Left err -> kf err
 
 {-# INLINE takeInt16BEWithFieldLength #-}
 
@@ -137,8 +140,8 @@ takeInt16BEWithFieldLength = do
 takeInt32BE :: Parser Int32
 takeInt32BE = Parser $ \idx bs kf ks ->
   case BinSer.decodeInt32BE idx bs of
-    Left err -> kf err
     Right v -> ks v (idx + 4) bs
+    Left err -> kf err
 
 {-# INLINE takeInt32BEWithFieldLength #-}
 
@@ -149,6 +152,20 @@ takeInt32BEWithFieldLength = do
   mi32 <- parsePgFieldWithAtMost4Bytes BinSer.TypeSize4
   pure $ fromIntegral <$> mi32
 
+{-# INLINE takeFloatBE #-}
+takeFloatBE :: Parser Float
+takeFloatBE = Parser $ \idx bs kf ks ->
+  case BinSer.decodeWord32BE idx bs of
+    Right v -> ks (castWord32ToFloat v) (idx + 4) bs
+    Left err -> kf err
+
+{-# INLINE takeDoubleBE #-}
+takeDoubleBE :: Parser Double
+takeDoubleBE = Parser $ \idx bs kf ks ->
+  case BinSer.decodeWord64BE idx bs of
+    Right v -> ks (castWord64ToDouble v) (idx + 8) bs
+    Left err -> kf err
+
 {-# INLINE takeInt64BEWithFieldLength #-}
 
 -- | Parses both a field length and the field itself, for
@@ -164,8 +181,8 @@ takeInt64BEWithFieldLength = do
 takeInt64BE :: Parser Int64
 takeInt64BE = Parser $ \idx bs kf ks ->
   case BinSer.decodeInt64BE idx bs of
-    Left err -> kf err
     Right v -> ks v (idx + 8) bs
+    Left err -> kf err
 
 {-# INLINE takeDataRow #-}
 
@@ -186,8 +203,8 @@ parsePgFieldWithAtMost4Bytes wdec =
   let dec = BinSer.decodePgFieldWithAtMost4Bytes wdec
    in Parser $ \idx bs kf ks ->
         case dec idx bs of
-          Left err -> kf err
           Right (v, restIdx) -> ks v restIdx bs
+          Left err -> kf err
 
 parseMany :: Parser a -> Parser [a]
 parseMany p = Parser $ \idx' bs' _kf ks -> let (vs, restIdx) = go idx' bs' in ks vs restIdx bs'

From 0be570453d28b532f22bd5fff02d9efa93a4ad57 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Wed, 12 Aug 2026 16:45:15 -0300
Subject: [PATCH 04/22] Specialized instance for UTCTime => clear benefits once
 again

---
 hpgsql/src/Hpgsql/Encoding.hs | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index b092942..0ffcf29 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -1081,6 +1081,26 @@ instance FromPgField UTCTime where
           parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
       Right $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
     Nothing -> Left "Cannot decode SQL null as the Haskell UTCTime type. Use a `Maybe UTCTime`"
+  singleFieldRowDecoder =
+    let fromNullable = \case
+          Nothing -> fail "Cannot decode SQL null as the Haskell UTCTime type. Use a `Maybe UTCTime`"
+          Just i -> pure i
+        utcTimeDecoder = do
+          len <- Parser.takeInt32BE
+          case len of
+            8 -> do
+              totalusecs <- Parser.takeInt64BE
+              let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
+                  parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
+              pure $ Just $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
+            _ -> pure Nothing
+     in RowDecoder
+          { fullRowDecoder = const $ utcTimeDecoder >>= fromNullable,
+            rowColumnsTypeCheck = \case
+              [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` [timestamptzOid])]
+              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+            numExpectedColumns = 1
+          }
 
 instance FromPgField (Unbounded UTCTime) where
   fieldDecoder = parsePgType [timestamptzOid] $ \case

From 68378ee48eea585c99e10875ad438080fdceaa55 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Wed, 12 Aug 2026 17:08:08 -0300
Subject: [PATCH 05/22] Tidy up a bit

---
 hpgsql/src/Hpgsql/Encoding.hs | 58 ++++++++++++++++-------------------
 1 file changed, 27 insertions(+), 31 deletions(-)

diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index 0ffcf29..d2a1914 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -181,6 +181,28 @@ singleField (FieldDecoder {..}) =
       numExpectedColumns = 1
     }
 
+{-# INLINE uniqueOidRowPa #-}
+uniqueOidRowPa :: Oid -> Parser.Parser a -> RowDecoder a
+uniqueOidRowPa tyoid p =
+  -- FromPgField instances that only accept PG values of a single PG type
+  -- are very dear to us because they allow a very important optimization:
+  -- their row decoders do not care about the `FieldInfo` argument, which
+  -- makes them inlinable by GHC at compile time (FieldInfo is only available
+  -- at run time).
+  -- These are key to produce compiled to code that almost compiles down to
+  -- a bunch of `peek` calls to a single ByteString decoding bytes into
+  -- typed values, to then call the Parser continuation, and repeat.
+  -- The only allocations (I think) when everything is inlined by this are the
+  -- decoded values themselves being boxed and the CPS Parser's ByteStringIdx
+  -- also being passed boxed between continuations.
+  RowDecoder
+    { fullRowDecoder = const p,
+      rowColumnsTypeCheck = \case
+        [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid == tyoid)]
+        _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+      numExpectedColumns = 1
+    }
+
 class FromPgField a where
   -- | A decoder that takes
   fieldDecoder :: FieldDecoder a
@@ -712,12 +734,6 @@ instance (ToPgField a, ToPgField b, ToPgField c, ToPgField d, ToPgField e, ToPgF
 instance (ToPgField a, ToPgField b, ToPgField c, ToPgField d, ToPgField e, ToPgField f, ToPgField g, ToPgField h, ToPgField i, ToPgField j, ToPgField k) => ToPgRow (a, b, c, d, e, f, g, h, i, j, k) where
   rowEncoder = divide (\(a, b, c, d, e, f, g, h, i, j, k) -> ((a, b, c, d, e, f), (g, h, i, j, k))) rowEncoder rowEncoder
 
--- instance (ToPgField a) => ToPgRow [a] where
---   rowEncoder = RowEncoder {
---     toPgParams = \xs -> concatMap toPgParams xs
---     , toTypeOids = \_ -> concatMap (\)
---   } $ \cols -> map (\v encodingContext -> let typOid = toTypeOid (Proxy @a) encodingContext in (typOid, toPgField encodingContext v)) cols
-
 -- | The OID for `Data.Int`, which is machine dependent.
 haskellIntOid :: Oid
 
@@ -996,17 +1012,10 @@ instance FromPgField Bool where
     Just bs -> Right $ bs == binaryTrue
     Nothing -> Left "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
   singleFieldRowDecoder =
-    let dec = Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1
-        word8ToBool = \case
+    let word8ToBool = \case
           Nothing -> fail "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
           Just w8 -> pure $ w8 == 1
-     in RowDecoder
-          { fullRowDecoder = const $ dec >>= word8ToBool,
-            rowColumnsTypeCheck = \case
-              [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid == boolOid)]
-              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-            numExpectedColumns = 1
-          }
+     in uniqueOidRowPa boolOid $ Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1 >>= word8ToBool
 
 instance FromPgField Char where
   fieldDecoder =
@@ -1094,13 +1103,7 @@ instance FromPgField UTCTime where
                   parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
               pure $ Just $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
             _ -> pure Nothing
-     in RowDecoder
-          { fullRowDecoder = const $ utcTimeDecoder >>= fromNullable,
-            rowColumnsTypeCheck = \case
-              [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` [timestamptzOid])]
-              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-            numExpectedColumns = 1
-          }
+     in uniqueOidRowPa timestamptzOid $ utcTimeDecoder >>= fromNullable
 
 instance FromPgField (Unbounded UTCTime) where
   fieldDecoder = parsePgType [timestamptzOid] $ \case
@@ -1172,17 +1175,10 @@ instance FromPgField Day where
       Right $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
     Nothing -> Left "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
   singleFieldRowDecoder =
-    let dec = Parser.takeInt32BEWithFieldLength
-        int32ToDay = \case
+    let int32ToDay = \case
           Nothing -> fail "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
           Just i32 -> let jd = fromIntegral i32 :: Integer in pure $ addJulianDurationClip (CalendarDiffDays 0 (jd - 13)) $ fromJulian 2000 01 01
-     in RowDecoder
-          { fullRowDecoder = const $ dec >>= int32ToDay,
-            rowColumnsTypeCheck = \case
-              [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid == dateOid)]
-              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-            numExpectedColumns = 1
-          }
+     in uniqueOidRowPa dateOid $ Parser.takeInt32BEWithFieldLength >>= int32ToDay
 
 instance FromPgField (Unbounded Day) where
   fieldDecoder = parsePgType [dateOid] $ \case

From feabc7835177d1249e3883542e2f2cb500cb1d31 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Thu, 13 Aug 2026 12:05:55 -0300
Subject: [PATCH 06/22] Slightly better understanding of inlining, clearer
 inlining boundaries

We're beginning to converge to row decoders not being inlined by
default, but their implementations/bodies being as inlined as possible,
which feels reasonable.

It remains to be seen if we can add a super-inlined version of row
decoders for users to choose from if they wish, and what the effects
are.
---
 TODO.md                                       |  4 ++
 hpgsql-tests/RowDecoderGhcCore.hs             | 55 ++++++++++++-------
 hpgsql/src/Hpgsql/Encoding.hs                 | 42 ++++++--------
 .../src/Hpgsql/Encoding/BinarySerializer.hs   |  4 +-
 hpgsql/src/Hpgsql/SimpleParser.hs             |  8 +++
 5 files changed, 66 insertions(+), 47 deletions(-)
 create mode 100644 TODO.md

diff --git a/TODO.md b/TODO.md
new file mode 100644
index 0000000..a53416c
--- /dev/null
+++ b/TODO.md
@@ -0,0 +1,4 @@
+- Test both `singleField fieldDecoder` and `singleFieldRowDecoder` for every type in our tests.
+- Make every FromPgField instance have a dedicated singleFieldRowDecoder override, change our benchmarks to exercise other types we're not, like `numeric` and `Float`
+- Investigate why overlapping (Maybe a) instance is better for record decoding but worse for Tuple decoding
+- Try to achieve a 100% inlined row decoder for a small record type
diff --git a/hpgsql-tests/RowDecoderGhcCore.hs b/hpgsql-tests/RowDecoderGhcCore.hs
index e53d8a5..2e431b9 100644
--- a/hpgsql-tests/RowDecoderGhcCore.hs
+++ b/hpgsql-tests/RowDecoderGhcCore.hs
@@ -2,37 +2,52 @@
 
 -- |
 -- This is not a real test module. It's just a type deriving `FromPgRow`
--- so we can look at GHC Core output.
+-- so we can look at GHC Core output. It's as small as we can make it to
+-- facilitate reading GHC Core.
 module RowDecoderGhcCore where
 
 import Data.Int (Int64)
 import Data.Text (Text)
 import Data.Time (Day, UTCTime)
 import GHC.Generics (Generic)
-import Hpgsql.Encoding (FromPgRow (..), fieldDecoder, genericFromPgRow, singleField)
-
-data BenchRow = BenchRow
-  { brId :: !Int,
-    brDate1 :: !Day,
-    brDate2 :: !Day,
-    brTimestamp1 :: !UTCTime,
-    brTimestamp2 :: !UTCTime,
-    brText1 :: !Text,
-    brText2 :: !Text,
-    brDouble1 :: !Double,
-    brDouble2 :: !Double,
-    brMaybeInt :: !(Maybe Int),
-    brMaybeText :: !(Maybe Text),
-    brMaybeDouble :: !(Maybe Double),
-    brMaybeDay :: !(Maybe Day)
+import Hpgsql.Encoding (FromPgRow (..), fieldDecoder, genericFromPgRow, singleField, singleFieldRowDecoder)
+
+-- | SmallRecord's purpose is to have a very small row decoder in GHC Core
+-- for understanding. Also, we expect one day to maybe reach a fully inlined
+-- row decoder that only peeks at bytes and allocates 3 values per row (one
+-- for each field), plus one `SmallRecord` per row.
+-- If we can get there, it'd be fabulous.
+data SmallRecord = SmallRecord
+  { smallId :: !Int,
+    smallDate :: !Day,
+    smallText :: !Int
   }
 
+instance FromPgRow SmallRecord where
+  rowDecoder = SmallRecord <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+
+-- data BenchRow = BenchRow
+--   { brId :: !Int,
+--     brDate1 :: !Day,
+--     brDate2 :: !Day,
+--     brTimestamp1 :: !UTCTime,
+--     brTimestamp2 :: !UTCTime,
+--     brText1 :: !Text,
+--     brText2 :: !Text,
+--     brDouble1 :: !Double,
+--     brDouble2 :: !Double,
+--     brMaybeInt :: !(Maybe Int),
+--     brMaybeText :: !(Maybe Text),
+--     brMaybeDouble :: !(Maybe Double),
+--     brMaybeDay :: !(Maybe Day)
+--   }
+
 -- Generically deriving section.
 
-deriving instance Generic BenchRow
+-- deriving instance Generic BenchRow
 
-instance FromPgRow BenchRow where
-  rowDecoder = genericFromPgRow
+-- instance FromPgRow BenchRow where
+--   rowDecoder = genericFromPgRow
 
 -- Hand-written applicative style deriving section.
 -- instance FromPgRow BenchRow where
diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index d2a1914..c928c1c 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -181,24 +181,25 @@ singleField (FieldDecoder {..}) =
       numExpectedColumns = 1
     }
 
-{-# INLINE uniqueOidRowPa #-}
-uniqueOidRowPa :: Oid -> Parser.Parser a -> RowDecoder a
-uniqueOidRowPa tyoid p =
-  -- FromPgField instances that only accept PG values of a single PG type
-  -- are very dear to us because they allow a very important optimization:
+{-# INLINE inlinableRowDecoder #-}
+inlinableRowDecoder :: [Oid] -> Parser.Parser a -> RowDecoder a
+inlinableRowDecoder tyoids p =
+  -- FromPgField instances whose decoders don't care about the OID of the PG type
+  -- being decoded are very dear to us because they allow a very important optimization:
   -- their row decoders do not care about the `FieldInfo` argument, which
   -- makes them inlinable by GHC at compile time (FieldInfo is only available
-  -- at run time).
+  -- at run time when the RowDescription message arrives for a given query).
   -- These are key to produce compiled to code that almost compiles down to
   -- a bunch of `peek` calls to a single ByteString decoding bytes into
   -- typed values, to then call the Parser continuation, and repeat.
   -- The only allocations (I think) when everything is inlined by this are the
   -- decoded values themselves being boxed and the CPS Parser's ByteStringIdx
-  -- also being passed boxed between continuations.
+  -- also being passed boxed between continuations (though reading GHC Core
+  -- is something I'm still learning).
   RowDecoder
     { fullRowDecoder = const p,
       rowColumnsTypeCheck = \case
-        [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid == tyoid)]
+        [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` tyoids)]
         _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
       numExpectedColumns = 1
     }
@@ -768,6 +769,7 @@ binaryIntDecoder typOid = \bs ->
     doesFit = maxBoundPgType <= fromIntegral (maxBound @a)
 
 -- | Specialized/performance-oriented Big-Endian binary decoder for Haskell's various IntXX types.
+{-# INLINE binaryIntSpecializedRowDecoder #-}
 binaryIntSpecializedRowDecoder :: Parser.Parser (Maybe Int)
 binaryIntSpecializedRowDecoder = do
   fieldLen <- Parser.takeInt32BE
@@ -813,17 +815,12 @@ instance FromPgField Int where
                 Nothing -> Left "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`",
         allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
       }
+  {-# NOINLINE singleFieldRowDecoder #-}
   singleFieldRowDecoder =
     let fromNullable = \case
           Nothing -> fail "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`"
           Just i -> pure i
-     in RowDecoder
-          { fullRowDecoder = const $ binaryIntSpecializedRowDecoder >>= fromNullable,
-            rowColumnsTypeCheck = \case
-              [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` haskellIntOids)]
-              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-            numExpectedColumns = 1
-          }
+     in inlinableRowDecoder haskellIntOids $ binaryIntSpecializedRowDecoder >>= fromNullable
 
 -- The instance below makes our Records benchmark faster and use less
 -- memory, but makes our Tuples benchmark slower. Worth investigating.
@@ -934,13 +931,7 @@ instance FromPgField Double where
             8 -> Just <$> Parser.takeDoubleBE
             4 -> Just . float2Double <$> Parser.takeFloatBE
             _ -> pure Nothing
-     in RowDecoder
-          { fullRowDecoder = const $ float4OrDouble8Decoder >>= fromNullable,
-            rowColumnsTypeCheck = \case
-              [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` [float8Oid, float4Oid])]
-              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-            numExpectedColumns = 1
-          }
+     in inlinableRowDecoder [float8Oid, float4Oid] $ float4OrDouble8Decoder >>= fromNullable
 
 -- | Allows you to specify a type (and other checks, possibly) for a `FieldDecoder`.
 -- This can be useful to ensure you're not accidentally decoding a different type.
@@ -1015,7 +1006,7 @@ instance FromPgField Bool where
     let word8ToBool = \case
           Nothing -> fail "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
           Just w8 -> pure $ w8 == 1
-     in uniqueOidRowPa boolOid $ Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1 >>= word8ToBool
+     in inlinableRowDecoder [boolOid] $ Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1 >>= word8ToBool
 
 instance FromPgField Char where
   fieldDecoder =
@@ -1103,7 +1094,7 @@ instance FromPgField UTCTime where
                   parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
               pure $ Just $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
             _ -> pure Nothing
-     in uniqueOidRowPa timestamptzOid $ utcTimeDecoder >>= fromNullable
+     in inlinableRowDecoder [timestamptzOid] $ utcTimeDecoder >>= fromNullable
 
 instance FromPgField (Unbounded UTCTime) where
   fieldDecoder = parsePgType [timestamptzOid] $ \case
@@ -1174,11 +1165,12 @@ instance FromPgField Day where
       jd <- BinSer.decodeInt32BE 0 bs
       Right $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
     Nothing -> Left "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
+  {-# NOINLINE singleFieldRowDecoder #-}
   singleFieldRowDecoder =
     let int32ToDay = \case
           Nothing -> fail "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
           Just i32 -> let jd = fromIntegral i32 :: Integer in pure $ addJulianDurationClip (CalendarDiffDays 0 (jd - 13)) $ fromJulian 2000 01 01
-     in uniqueOidRowPa dateOid $ Parser.takeInt32BEWithFieldLength >>= int32ToDay
+     in inlinableRowDecoder [dateOid] $ Parser.takeInt32BEWithFieldLength >>= int32ToDay
 
 instance FromPgField (Unbounded Day) where
   fieldDecoder = parsePgType [dateOid] $ \case
diff --git a/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs b/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
index 4aab61a..a1274b2 100644
--- a/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
+++ b/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
@@ -178,13 +178,13 @@ decodeDataRow idx bs@(InternalBS.BS _bytesPtr len) =
       | len >= 1 + lenFullMsg + idx.idx = Right $ ByteStringIdx $ 1 + lenFullMsg + idx.idx
       | otherwise = Left "Less than enough bytes to decode a full DataRow"
 
-{-# INLINE decodePgFieldWithAtMost4Bytes #-}
-
 data WordDecoding a where
   TypeSize1 :: WordDecoding Word8
   TypeSize2 :: WordDecoding Word16
   TypeSize4 :: WordDecoding Word32
 
+{-# INLINE decodePgFieldWithAtMost4Bytes #-}
+
 -- | A specialized decoder that decoders a query result's
 -- field's contents, but only for PG fields at most 4 bytes long and
 -- at least 1 byte long (so no text or void types, for example).
diff --git a/hpgsql/src/Hpgsql/SimpleParser.hs b/hpgsql/src/Hpgsql/SimpleParser.hs
index 659f9e1..23942e4 100644
--- a/hpgsql/src/Hpgsql/SimpleParser.hs
+++ b/hpgsql/src/Hpgsql/SimpleParser.hs
@@ -33,6 +33,7 @@ module Hpgsql.SimpleParser
   )
 where
 
+import Control.Applicative (Alternative (..))
 import Data.ByteString (ByteString)
 import qualified Data.ByteString as BS
 import Data.Int (Int16, Int32, Int64)
@@ -73,6 +74,13 @@ instance Applicative Parser where
     pf idx bs kf (\f bs' idx' -> pa bs' idx' kf (\a bs'' idx'' -> ks (f a) bs'' idx''))
   {-# INLINE (<*>) #-}
 
+instance Alternative Parser where
+  empty = fail "empty Alternative"
+  {-# INLINE empty #-}
+  Parser p1 <|> Parser p2 = Parser $ \idx bs kf ks ->
+    p1 idx bs (\_ -> p2 idx bs kf ks) ks
+  {-# INLINE (<|>) #-}
+
 instance Monad Parser where
   return = pure
   {-# INLINE return #-}

From 7030fd56a0087046361e0fc40d398720b41ebbf3 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Thu, 13 Aug 2026 16:00:09 -0300
Subject: [PATCH 07/22] Add `inlinedSingleFieldDecoder`

The boundary here is very nice: users that want to avoid too much code
bloat can use the non-inlined decoders, and those that want to max out
performance can use the inlined decoders.
---
 TODO.md                           |   1 +
 hpgsql-benchmarks/src/Main.hs     |   6 +-
 hpgsql-tests/RowDecoderGhcCore.hs |  29 ++++---
 hpgsql/src/Hpgsql/Encoding.hs     | 140 +++++++++++++++++++-----------
 4 files changed, 115 insertions(+), 61 deletions(-)

diff --git a/TODO.md b/TODO.md
index a53416c..e3e3aa8 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,4 +1,5 @@
 - Test both `singleField fieldDecoder` and `singleFieldRowDecoder` for every type in our tests.
 - Make every FromPgField instance have a dedicated singleFieldRowDecoder override, change our benchmarks to exercise other types we're not, like `numeric` and `Float`
 - Investigate why overlapping (Maybe a) instance is better for record decoding but worse for Tuple decoding
+  - Revert things: derive the overlapping (Maybe a) instance, derive the `FromPgField a` using that under the hood.
 - Try to achieve a 100% inlined row decoder for a small record type
diff --git a/hpgsql-benchmarks/src/Main.hs b/hpgsql-benchmarks/src/Main.hs
index 139bd82..41a7f0e 100644
--- a/hpgsql-benchmarks/src/Main.hs
+++ b/hpgsql-benchmarks/src/Main.hs
@@ -46,6 +46,7 @@ import Hpgsql.Connection (renderLibpqConnectionString)
 import qualified Hpgsql.Connection
 import qualified Hpgsql.Connection as Hpgsql
 import qualified Hpgsql.Copy
+import Hpgsql.Encoding (FromPgField (inlinedSingleFieldRowDecoder))
 import qualified Hpgsql.Encoding as Hpgsql
 import qualified Hpgsql.Query as Hpgsql
 import qualified Hpgsql.Types as Hpgsql
@@ -84,7 +85,10 @@ data BenchRow = BenchRow
     brMaybeDay :: !(Maybe Day)
   }
   deriving stock (Generic, Show, Eq)
-  deriving anyclass (NFData, Hpgsql.FromPgRow, PGSimple.FromRow)
+  deriving anyclass (NFData, PGSimple.FromRow)
+
+instance Hpgsql.FromPgRow BenchRow where
+  rowDecoder = BenchRow <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 data HasqlBenchRow = HasqlBenchRow
   { hbrId :: !Int32,
diff --git a/hpgsql-tests/RowDecoderGhcCore.hs b/hpgsql-tests/RowDecoderGhcCore.hs
index 2e431b9..0e1fab9 100644
--- a/hpgsql-tests/RowDecoderGhcCore.hs
+++ b/hpgsql-tests/RowDecoderGhcCore.hs
@@ -10,21 +10,28 @@ import Data.Int (Int64)
 import Data.Text (Text)
 import Data.Time (Day, UTCTime)
 import GHC.Generics (Generic)
-import Hpgsql.Encoding (FromPgRow (..), fieldDecoder, genericFromPgRow, singleField, singleFieldRowDecoder)
+import Hpgsql.Encoding (FromPgField (..), FromPgRow (..), genericFromPgRow, singleField)
 
--- | SmallRecord's purpose is to have a very small row decoder in GHC Core
--- for understanding. Also, we expect one day to maybe reach a fully inlined
+-- | BestCaseScenarioRecord's purpose is to have a very small row decoder in GHC Core
+-- for my own understanding/comprehension of what a RowDecoder gets compiled to
+-- in the best case scenario. Also, we expect one day to maybe reach a fully inlined
 -- row decoder that only peeks at bytes and allocates 3 values per row (one
--- for each field), plus one `SmallRecord` per row.
--- If we can get there, it'd be fabulous.
-data SmallRecord = SmallRecord
-  { smallId :: !Int,
-    smallDate :: !Day,
-    smallText :: !Int
+-- for each field), plus one `BestCaseScenarioRecord` per row.
+-- In the GHC Core of this module (use `run ghc-core` to output it), it helps to:
+-- - Look for the Record constructor and grep for it to find where the RowDecoder
+--   invokes it, only to find where the RowDecoder is.
+-- - Grep for numbers that exist in the decoders' implementation, such as 8#, 13#, 4#.
+--   These are a strong indicator that each decoder was inlined into the RowDecoder.
+-- There is still unnecessary allocations/boxing even with full inlining, but maybe
+-- one day we'll find a way to get rid of all of them.
+data BestCaseScenarioRecord = BestCaseScenarioRecord
+  { bcsId :: !Int,
+    bcsDate :: !Day,
+    bcsText :: !Int
   }
 
-instance FromPgRow SmallRecord where
-  rowDecoder = SmallRecord <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+instance FromPgRow BestCaseScenarioRecord where
+  rowDecoder = BestCaseScenarioRecord <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 -- data BenchRow = BenchRow
 --   { brId :: !Int,
diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index c928c1c..d767f62 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -216,6 +216,14 @@ class FromPgField a where
   singleFieldRowDecoder :: RowDecoder a
   singleFieldRowDecoder = singleField fieldDecoder
 
+  -- | This is just like `singleFieldDecoder`, but it inlines into your
+  -- `FromPgRow` instances aggressively. This will increase code size and
+  -- possibly compilation times somewhat, but in some cases it can make row decoders
+  -- compile down to a ByteString-peeking implementation with much fewer
+  -- allocations that can be ~10% faster than the other.
+  inlinedSingleFieldRowDecoder :: RowDecoder a
+  inlinedSingleFieldRowDecoder = singleFieldRowDecoder
+
 class FromPgRow a where
   rowDecoder :: RowDecoder a
   default rowDecoder :: (Generic a, ProductTypeDecoder (Rep a)) => RowDecoder a
@@ -768,20 +776,6 @@ binaryIntDecoder typOid = \bs ->
       | otherwise = error "Bug in Hpgsql. Decoding binary integral type not an int2, int4 or int8"
     doesFit = maxBoundPgType <= fromIntegral (maxBound @a)
 
--- | Specialized/performance-oriented Big-Endian binary decoder for Haskell's various IntXX types.
-{-# INLINE binaryIntSpecializedRowDecoder #-}
-binaryIntSpecializedRowDecoder :: Parser.Parser (Maybe Int)
-binaryIntSpecializedRowDecoder = do
-  fieldLen <- Parser.takeInt32BE
-  -- TODO: We're assuming `Int` is always 64 bits, so 64bit CPUs? Is that ok?
-  -- TODO: Is there a way to optimistically assume <=4 bytes and use our custom new parser?
-  case fieldLen of
-    4 -> Just . fromIntegral <$> Parser.takeInt32BE
-    (-1) -> pure Nothing
-    8 -> Just . fromIntegral <$> Parser.takeInt64BE
-    2 -> Just . fromIntegral <$> Parser.takeInt16BE
-    _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
-
 binaryFloat4Decoder :: ByteString -> Float
 binaryFloat4Decoder = castWord32ToFloat . either error id . BinSer.decodeWord32BE 0
 
@@ -805,6 +799,19 @@ instance FromPgField () where
         allowedPgTypes = (== voidOid) . fieldTypeOid
       }
 
+{-# INLINE intRowDecoder #-}
+intRowDecoder =
+  inlinableRowDecoder haskellIntOids $ do
+    fieldLen <- Parser.takeInt32BE
+    -- TODO: We're assuming `Int` is always 64 bits, so 64bit CPUs? Is that ok?
+    -- TODO: Is there a way to optimistically assume <=4 bytes and use our custom new parser?
+    case fieldLen of
+      4 -> fromIntegral <$> Parser.takeInt32BE
+      (-1) -> fail "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`"
+      8 -> fromIntegral <$> Parser.takeInt64BE
+      2 -> fromIntegral <$> Parser.takeInt16BE
+      _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
+
 instance FromPgField Int where
   fieldDecoder =
     FieldDecoder
@@ -816,11 +823,9 @@ instance FromPgField Int where
         allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder =
-    let fromNullable = \case
-          Nothing -> fail "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`"
-          Just i -> pure i
-     in inlinableRowDecoder haskellIntOids $ binaryIntSpecializedRowDecoder >>= fromNullable
+  singleFieldRowDecoder = intRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = intRowDecoder
 
 -- The instance below makes our Records benchmark faster and use less
 -- memory, but makes our Tuples benchmark slower. Worth investigating.
@@ -909,6 +914,19 @@ instance FromPgField Float where
     Just bs -> Right $ binaryFloat4Decoder bs
     Nothing -> Left "Cannot decode SQL null as the Haskell Float type. Use a `Maybe Float`"
 
+{-# INLINE doubleRowDecoder #-}
+doubleRowDecoder =
+  let fromNullable = \case
+        Nothing -> fail "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`"
+        Just i -> pure i
+      float4OrDouble8Decoder = do
+        len <- Parser.takeInt32BE
+        case len of
+          8 -> Just <$> Parser.takeDoubleBE
+          4 -> Just . float2Double <$> Parser.takeFloatBE
+          _ -> fail "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`"
+   in inlinableRowDecoder [float8Oid, float4Oid] $ float4OrDouble8Decoder >>= fromNullable
+
 instance FromPgField Double where
   fieldDecoder =
     FieldDecoder
@@ -921,17 +939,10 @@ instance FromPgField Double where
                 Nothing -> Left "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`",
         allowedPgTypes = (`elem` [float8Oid, float4Oid]) . fieldTypeOid
       }
-  singleFieldRowDecoder =
-    let fromNullable = \case
-          Nothing -> fail "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`"
-          Just i -> pure i
-        float4OrDouble8Decoder = do
-          len <- Parser.takeInt32BE
-          case len of
-            8 -> Just <$> Parser.takeDoubleBE
-            4 -> Just . float2Double <$> Parser.takeFloatBE
-            _ -> pure Nothing
-     in inlinableRowDecoder [float8Oid, float4Oid] $ float4OrDouble8Decoder >>= fromNullable
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = doubleRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = doubleRowDecoder
 
 -- | Allows you to specify a type (and other checks, possibly) for a `FieldDecoder`.
 -- This can be useful to ensure you're not accidentally decoding a different type.
@@ -1038,11 +1049,28 @@ instance FromPgField LBS.ByteString where
     Just bs -> Right $ LBS.fromStrict bs
     Nothing -> Left "Cannot decode SQL null as the Haskell ByteString type. Use a `Maybe ByteString`"
 
+{-# INLINE textDecoder #-}
+textDecoder =
+  let fromNullable = \case
+        Nothing -> fail "Cannot decode SQL null as the Haskell Text type. Use a `Maybe Text`"
+        Just i -> pure i
+      rp = do
+        len <- Parser.takeInt32BE
+        if len >= 0
+          -- TODO: Use some faster unsafeDecodeUtf8 function?
+          then Just . decodeUtf8 <$> Parser.take (fromIntegral len)
+          else pure Nothing
+   in inlinableRowDecoder [textOid, varcharOid, nameOid] $ rp >>= fromNullable
+
 instance FromPgField Text where
   fieldDecoder = parsePgType [textOid, varcharOid, nameOid] $ \case
     Just bs -> Right $ decodeUtf8 bs
     -- TODO: Use some faster unsafeDecodeUtf8 function?
     Nothing -> Left "Cannot decode SQL null as the Haskell Text type. Use a `Maybe Text`"
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = textDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = textDecoder
 
 instance FromPgField LT.Text where
   fieldDecoder = parsePgType [textOid, varcharOid, nameOid] $ \case
@@ -1072,6 +1100,22 @@ instance FromPgField (CI LT.Text) where
 instance FromPgField (CI String) where
   fieldDecoder = typeFieldDecoder (typeMustBeNamed "citext") $ CI.mk <$> fieldDecoder
 
+{-# INLINE utcTimeRowDecoder #-}
+utcTimeRowDecoder =
+  let fromNullable = \case
+        Nothing -> fail "Cannot decode SQL null as the Haskell UTCTime type. Use a `Maybe UTCTime`"
+        Just i -> pure i
+      utcTimeDecoder = do
+        len <- Parser.takeInt32BE
+        case len of
+          8 -> do
+            totalusecs <- Parser.takeInt64BE
+            let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
+                parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
+            pure $ Just $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
+          _ -> pure Nothing
+   in inlinableRowDecoder [timestamptzOid] $ utcTimeDecoder >>= fromNullable
+
 instance FromPgField UTCTime where
   fieldDecoder = parsePgType [timestamptzOid] $ \case
     Just bs -> do
@@ -1081,20 +1125,13 @@ instance FromPgField UTCTime where
           parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
       Right $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
     Nothing -> Left "Cannot decode SQL null as the Haskell UTCTime type. Use a `Maybe UTCTime`"
-  singleFieldRowDecoder =
-    let fromNullable = \case
-          Nothing -> fail "Cannot decode SQL null as the Haskell UTCTime type. Use a `Maybe UTCTime`"
-          Just i -> pure i
-        utcTimeDecoder = do
-          len <- Parser.takeInt32BE
-          case len of
-            8 -> do
-              totalusecs <- Parser.takeInt64BE
-              let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
-                  parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-              pure $ Just $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-            _ -> pure Nothing
-     in inlinableRowDecoder [timestamptzOid] $ utcTimeDecoder >>= fromNullable
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = utcTimeRowDecoder
+  {-# NOINLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = utcTimeRowDecoder
+
+-- {-# INLINE inlinedSingleFieldRowDecoder #-}
+-- inlinedSingleFieldRowDecoder = doubleRowDecoder
 
 instance FromPgField (Unbounded UTCTime) where
   fieldDecoder = parsePgType [timestamptzOid] $ \case
@@ -1156,6 +1193,13 @@ instance FromPgField TimeOfDay where
       Right $ timeToTimeOfDay $ picosecondsToDiffTime $ fromIntegral usecs * 1_000_000
     Nothing -> Left "Cannot decode SQL null as the Haskell TimeOfDay type. Use a `Maybe TimeOfDay`"
 
+{-# INLINE dayRowDecoder #-}
+dayRowDecoder =
+  let int32ToDay = \case
+        Nothing -> fail "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
+        Just i32 -> let jd = fromIntegral i32 :: Integer in pure $ addJulianDurationClip (CalendarDiffDays 0 (jd - 13)) $ fromJulian 2000 01 01
+   in inlinableRowDecoder [dateOid] $ Parser.takeInt32BEWithFieldLength >>= int32ToDay
+
 instance FromPgField Day where
   fieldDecoder = parsePgType [dateOid] $ \case
     Just bs -> do
@@ -1166,11 +1210,9 @@ instance FromPgField Day where
       Right $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
     Nothing -> Left "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
   {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder =
-    let int32ToDay = \case
-          Nothing -> fail "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
-          Just i32 -> let jd = fromIntegral i32 :: Integer in pure $ addJulianDurationClip (CalendarDiffDays 0 (jd - 13)) $ fromJulian 2000 01 01
-     in inlinableRowDecoder [dateOid] $ Parser.takeInt32BEWithFieldLength >>= int32ToDay
+  singleFieldRowDecoder = dayRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = dayRowDecoder
 
 instance FromPgField (Unbounded Day) where
   fieldDecoder = parsePgType [dateOid] $ \case

From 3bf360bb711297fe7b90fdeb43e4ea2e692cf610 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Fri, 14 Aug 2026 12:05:34 -0300
Subject: [PATCH 08/22] More comprehensive coverage of types in benchmarks,
 more specialized row decoders

---
 hpgsql-benchmarks/hpgsql-benchmarks.cabal |   1 +
 hpgsql-benchmarks/src/Main.hs             |  67 +++++++++----
 hpgsql/src/Hpgsql/Encoding.hs             | 116 ++++++++++++++++++----
 hpgsql/src/Hpgsql/SimpleParser.hs         |  12 ++-
 4 files changed, 156 insertions(+), 40 deletions(-)

diff --git a/hpgsql-benchmarks/hpgsql-benchmarks.cabal b/hpgsql-benchmarks/hpgsql-benchmarks.cabal
index aa34959..d7269ae 100644
--- a/hpgsql-benchmarks/hpgsql-benchmarks.cabal
+++ b/hpgsql-benchmarks/hpgsql-benchmarks.cabal
@@ -72,6 +72,7 @@ executable hpgsql-benchmarks
     , hspec-expectations
     , postgresql-simple
     , resourcet
+    , scientific
     , statistics
     , stm
     , streaming
diff --git a/hpgsql-benchmarks/src/Main.hs b/hpgsql-benchmarks/src/Main.hs
index 41a7f0e..58420e4 100644
--- a/hpgsql-benchmarks/src/Main.hs
+++ b/hpgsql-benchmarks/src/Main.hs
@@ -24,6 +24,7 @@ import Criterion.Measurement.Types
   )
 import qualified Data.ByteString.Char8 as BS8
 import Data.Int (Int32, Int64)
+import Data.Scientific (Scientific)
 import Data.String (IsString)
 import Data.Text (Text)
 import qualified Data.Text as Text
@@ -82,13 +83,18 @@ data BenchRow = BenchRow
     brMaybeInt :: !(Maybe Int),
     brMaybeText :: !(Maybe Text),
     brMaybeDouble :: !(Maybe Double),
-    brMaybeDay :: !(Maybe Day)
+    brMaybeDay :: !(Maybe Day),
+    brNumeric :: !Scientific,
+    brFloat :: !Float,
+    brBool1 :: Bool,
+    brBool2 :: Bool
   }
   deriving stock (Generic, Show, Eq)
-  deriving anyclass (NFData, PGSimple.FromRow)
+  deriving anyclass (NFData, Hpgsql.FromPgRow, PGSimple.FromRow)
 
-instance Hpgsql.FromPgRow BenchRow where
-  rowDecoder = BenchRow <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
+fullyInlinedBenchRowDecoder :: Hpgsql.RowDecoder BenchRow
+fullyInlinedBenchRowDecoder =
+  BenchRow <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 data HasqlBenchRow = HasqlBenchRow
   { hbrId :: !Int32,
@@ -103,7 +109,11 @@ data HasqlBenchRow = HasqlBenchRow
     hbrMaybeInt :: !(Maybe Int32),
     hbrMaybeText :: !(Maybe Text),
     hbrMaybeDouble :: !(Maybe Double),
-    hbrMaybeDay :: !(Maybe Day)
+    hbrMaybeDay :: !(Maybe Day),
+    hbrNumeric :: !Scientific,
+    hbrFloat :: !Float,
+    hbrBool1 :: Bool,
+    hbrBool2 :: Bool
   }
   deriving stock (Generic, Show, Eq)
   deriving anyclass (NFData)
@@ -169,12 +179,14 @@ main = do
 
   statsBefore <- getRTSStats
   hspecWith defaultConfig {configFormat = Just (formatterToFormat silent)} $ do
+    let sql17 = "SELECT g, ('2000-01-01'::date + g::int4), ('2000-06-15'::date + g::int4), ('2000-01-01T00:00:00Z'::timestamptz + g * interval '1 second'), ('2020-06-15T12:00:00Z'::timestamptz + g * interval '1 minute'), 'row-' || g::text, 'item-' || g::text, g::float8 * 1.5, g::float8 * 2.5, NULL::int4, NULL::text, NULL::float8, NULL::date, g::numeric, g::float4, g%2=0, g%2=1 FROM generate_series(1,$1) g"
+        sql17Simple = "SELECT g, ('2000-01-01'::date + g::int4), ('2000-06-15'::date + g::int4), ('2000-01-01T00:00:00Z'::timestamptz + g * interval '1 second'), ('2020-06-15T12:00:00Z'::timestamptz + g * interval '1 minute'), 'row-' || g::text, 'item-' || g::text, g::float8 * 1.5, g::float8 * 2.5, NULL::int4, NULL::text, NULL::float8, NULL::date, g::numeric, g::float4, g%2=0, g%2=1 FROM generate_series(1,?) g"
+        sql13 = "SELECT g, ('2000-01-01'::date + g::int4), ('2000-06-15'::date + g::int4), ('2000-01-01T00:00:00Z'::timestamptz + g * interval '1 second'), ('2020-06-15T12:00:00Z'::timestamptz + g * interval '1 minute'), 'row-' || g::text, 'item-' || g::text, g::float8 * 1.5, g::float8 * 2.5, NULL::int4, NULL::text, NULL::float8, NULL::date FROM generate_series(1,$1) g"
+        sql13Simple = "SELECT g, ('2000-01-01'::date + g::int4), ('2000-06-15'::date + g::int4), ('2000-01-01T00:00:00Z'::timestamptz + g * interval '1 second'), ('2020-06-15T12:00:00Z'::timestamptz + g * interval '1 minute'), 'row-' || g::text, 'item-' || g::text, g::float8 * 1.5, g::float8 * 2.5, NULL::int4, NULL::text, NULL::float8, NULL::date FROM generate_series(1,?) g"
     describe "Parsing 13-column rows into a List" $ do
-      let sql = "SELECT g, ('2000-01-01'::date + g::int4), ('2000-06-15'::date + g::int4), ('2000-01-01T00:00:00Z'::timestamptz + g * interval '1 second'), ('2020-06-15T12:00:00Z'::timestamptz + g * interval '1 minute'), 'row-' || g::text, 'item-' || g::text, g::float8 * 1.5, g::float8 * 2.5, NULL::int4, NULL::text, NULL::float8, NULL::date FROM generate_series(1,$1) g"
-          pgSimpleSql = "SELECT g, ('2000-01-01'::date + g::int4), ('2000-06-15'::date + g::int4), ('2000-01-01T00:00:00Z'::timestamptz + g * interval '1 second'), ('2020-06-15T12:00:00Z'::timestamptz + g * interval '1 minute'), 'row-' || g::text, 'item-' || g::text, g::float8 * 1.5, g::float8 * 2.5, NULL::int4, NULL::text, NULL::float8, NULL::date FROM generate_series(1,?) g"
-          hasqlListStmt =
+      let hasqlListStmt =
             HasqlStmt.Statement
-              sql
+              sql13
               (HasqlEnc.param (HasqlEnc.nonNullable HasqlEnc.int4))
               ( HasqlDec.rowList
                   ( (,,,,,,,,,,,,)
@@ -196,7 +208,7 @@ main = do
               True
           hasqlRecordListStmt =
             HasqlStmt.Statement
-              sql
+              sql17
               (HasqlEnc.param (HasqlEnc.nonNullable HasqlEnc.int4))
               ( HasqlDec.rowList
                   ( HasqlBenchRow
@@ -213,15 +225,19 @@ main = do
                       <*> HasqlDec.column (HasqlDec.nullable HasqlDec.text)
                       <*> HasqlDec.column (HasqlDec.nullable HasqlDec.float8)
                       <*> HasqlDec.column (HasqlDec.nullable HasqlDec.date)
+                      <*> HasqlDec.column (HasqlDec.nonNullable HasqlDec.numeric)
+                      <*> HasqlDec.column (HasqlDec.nonNullable HasqlDec.float4)
+                      <*> HasqlDec.column (HasqlDec.nonNullable HasqlDec.bool)
+                      <*> HasqlDec.column (HasqlDec.nonNullable HasqlDec.bool)
                   )
               )
               True
-      forM_ [10_000 :: Int, 100_000] $ \n -> do
+      forM_ [100_000 :: Int] $ \n -> do
         it ("hpgsql Tuple List (" ++ show n ++ " rows)") $
           void $
             bench ("hpgsql Tuple List (" ++ show n ++ " rows)") $
               withMultipleConnections numConcurrentConnections hpgsqlConnect Hpgsql.Connection.closeGracefully $ \conn -> do
-                Hpgsql.queryWith (Hpgsql.rowDecoder @(Int, Day, Day, UTCTime, UTCTime, Text, Text, Double, Double, Maybe Int, Maybe Text, Maybe Double, Maybe Day)) conn (Hpgsql.mkQuery sql (Hpgsql.Only n))
+                Hpgsql.queryWith (Hpgsql.rowDecoder @(Int, Day, Day, UTCTime, UTCTime, Text, Text, Double, Double, Maybe Int, Maybe Text, Maybe Double, Maybe Day)) conn (Hpgsql.mkQuery sql13 (Hpgsql.Only n))
         it ("hasql Tuple List (" ++ show n ++ " rows)") $
           void $
             bench ("hasql Tuple List (" ++ show n ++ " rows)") $
@@ -232,12 +248,12 @@ main = do
           void $
             bench ("postgresql-simple Tuple List (" ++ show n ++ " rows)") $
               withMultipleConnections numConcurrentConnections pgSimpleConnect PGSimple.close $ \pgSimpleConn -> do
-                PGSimple.query @_ @(Int, Day, Day, UTCTime, UTCTime, Text, Text, Double, Double, Maybe Int, Maybe Text, Maybe Double, Maybe Day) pgSimpleConn pgSimpleSql (PGSimple.Only n)
+                PGSimple.query @_ @(Int, Day, Day, UTCTime, UTCTime, Text, Text, Double, Double, Maybe Int, Maybe Text, Maybe Double, Maybe Day) pgSimpleConn sql13Simple (PGSimple.Only n)
         it ("hpgsql Record List (" ++ show n ++ " rows)") $
           void $
             bench ("hpgsql Record List (" ++ show n ++ " rows)") $
               withMultipleConnections numConcurrentConnections hpgsqlConnect Hpgsql.Connection.closeGracefully $ \conn -> do
-                Hpgsql.queryWith (Hpgsql.rowDecoder @BenchRow) conn (Hpgsql.mkQuery sql (Hpgsql.Only n))
+                Hpgsql.queryWith (Hpgsql.rowDecoder @BenchRow) conn (Hpgsql.mkQuery sql17 (Hpgsql.Only n))
         it ("hasql Record List (" ++ show n ++ " rows)") $
           void $
             bench ("hasql Record List (" ++ show n ++ " rows)") $
@@ -248,40 +264,47 @@ main = do
           void $
             bench ("postgresql-simple Record List (" ++ show n ++ " rows)") $
               withMultipleConnections numConcurrentConnections pgSimpleConnect PGSimple.close $ \pgSimpleConn -> do
-                PGSimple.query @_ @BenchRow pgSimpleConn pgSimpleSql (PGSimple.Only n)
+                PGSimple.query @_ @BenchRow pgSimpleConn sql17Simple (PGSimple.Only n)
     describe "Parsing 13-column rows in streaming fashion" $ do
-      let sql = "SELECT g, ('2000-01-01'::date + g::int4), ('2000-06-15'::date + g::int4), ('2000-01-01T00:00:00Z'::timestamptz + g * interval '1 second'), ('2020-06-15T12:00:00Z'::timestamptz + g * interval '1 minute'), 'row-' || g::text, 'item-' || g::text, g::float8 * 1.5, g::float8 * 2.5, NULL::int4, NULL::text, NULL::float8, NULL::date FROM generate_series(1,$1) g"
-      forM_ [10_000 :: Int, 100_000] $ \n -> do
+      forM_ [100_000 :: Int] $ \n -> do
         it ("hpgsql Tuple Stream (" ++ show n ++ " rows)") $
           void $
             bench ("hpgsql Tuple Stream (" ++ show n ++ " rows)") $
               withMultipleConnections numConcurrentConnections hpgsqlConnect Hpgsql.Connection.closeGracefully $ \conn -> do
-                res <- Hpgsql.querySWith (Hpgsql.rowDecoder @(Int, Day, Day, UTCTime, UTCTime, Text, Text, Double, Double, Maybe Int, Maybe Text, Maybe Double, Maybe Day)) conn (Hpgsql.mkQuery sql (Hpgsql.Only n))
+                res <- Hpgsql.querySWith (Hpgsql.rowDecoder @(Int, Day, Day, UTCTime, UTCTime, Text, Text, Double, Double, Maybe Int, Maybe Text, Maybe Double, Maybe Day)) conn (Hpgsql.mkQuery sql13 (Hpgsql.Only n))
                 S.effects res
         it ("streaming-postgresql-simple Tuple Stream (" ++ show n ++ " rows)") $
           void $
             bench ("streaming-postgresql-simple Tuple Stream (" ++ show n ++ " rows)") $
               withMultipleConnections numConcurrentConnections pgSimpleConnect PGSimple.close $ \pgSimpleConn -> do
                 runResourceT @IO $ do
-                  let res :: Stream (Of (Int, Day, Day, UTCTime, UTCTime, Text, Text, Double, Double, Maybe Int, Maybe Text, Maybe Double, Maybe Day)) (ResourceT IO) () = StreamingPostgresSimple.query pgSimpleConn "SELECT g, ('2000-01-01'::date + g::int4), ('2000-06-15'::date + g::int4), ('2000-01-01T00:00:00Z'::timestamptz + g * interval '1 second'), ('2020-06-15T12:00:00Z'::timestamptz + g * interval '1 minute'), 'row-' || g::text, 'item-' || g::text, g::float8 * 1.5, g::float8 * 2.5, NULL::int4, NULL::text, NULL::float8, NULL::date FROM generate_series(1,?) g" (PGSimple.Only n)
+                  let res :: Stream (Of (Int, Day, Day, UTCTime, UTCTime, Text, Text, Double, Double, Maybe Int, Maybe Text, Maybe Double, Maybe Day)) (ResourceT IO) () = StreamingPostgresSimple.query pgSimpleConn sql13Simple (PGSimple.Only n)
                   S.effects res
         it ("postgresql-simple Tuple fold (" ++ show n ++ " rows)") $
           void $
             bench ("postgresql-simple Tuple fold (" ++ show n ++ " rows)") $
               withMultipleConnections numConcurrentConnections pgSimpleConnect PGSimple.close $ \pgSimpleConn -> do
-                PGSimple.fold pgSimpleConn "SELECT g, ('2000-01-01'::date + g::int4), ('2000-06-15'::date + g::int4), ('2000-01-01T00:00:00Z'::timestamptz + g * interval '1 second'), ('2020-06-15T12:00:00Z'::timestamptz + g * interval '1 minute'), 'row-' || g::text, 'item-' || g::text, g::float8 * 1.5, g::float8 * 2.5, NULL::int4, NULL::text, NULL::float8, NULL::date FROM generate_series(1,?) g" (PGSimple.Only n) () (\() (!_ :: (Int, Day, Day, UTCTime, UTCTime, Text, Text, Double, Double, Maybe Int, Maybe Text, Maybe Double, Maybe Day)) -> pure ())
+                PGSimple.fold pgSimpleConn sql13Simple (PGSimple.Only n) () (\() (!_ :: (Int, Day, Day, UTCTime, UTCTime, Text, Text, Double, Double, Maybe Int, Maybe Text, Maybe Double, Maybe Day)) -> pure ())
+    describe "Parsing 17-column rows in streaming fashion" $ do
+      forM_ [100_000 :: Int] $ \n -> do
         it ("hpgsql Record Stream (" ++ show n ++ " rows)") $
           void $
             bench ("hpgsql Record Stream (" ++ show n ++ " rows)") $ do
               withMultipleConnections numConcurrentConnections hpgsqlConnect Hpgsql.Connection.closeGracefully $ \conn -> do
-                res <- Hpgsql.querySWith (Hpgsql.rowDecoder @BenchRow) conn (Hpgsql.mkQuery sql (Hpgsql.Only n))
+                res <- Hpgsql.querySWith (Hpgsql.rowDecoder @BenchRow) conn (Hpgsql.mkQuery sql17 (Hpgsql.Only n))
+                S.effects res
+        it ("hpgsql Record Stream (" ++ show n ++ " rows, fully inlined row decoder)") $
+          void $
+            bench ("hpgsql Record Stream (" ++ show n ++ " rows, fully inlined row decoder)") $ do
+              withMultipleConnections numConcurrentConnections hpgsqlConnect Hpgsql.Connection.closeGracefully $ \conn -> do
+                res <- Hpgsql.querySWith fullyInlinedBenchRowDecoder conn (Hpgsql.mkQuery sql17 (Hpgsql.Only n))
                 S.effects res
         it ("streaming-postgresql-simple Record Stream (" ++ show n ++ " rows)") $
           void $
             bench ("streaming-postgresql-simple Record Stream (" ++ show n ++ " rows)") $
               withMultipleConnections numConcurrentConnections pgSimpleConnect PGSimple.close $ \pgSimpleConn -> do
                 runResourceT @IO $ do
-                  let res :: Stream (Of BenchRow) (ResourceT IO) () = StreamingPostgresSimple.query pgSimpleConn "SELECT g, ('2000-01-01'::date + g::int4), ('2000-06-15'::date + g::int4), ('2000-01-01T00:00:00Z'::timestamptz + g * interval '1 second'), ('2020-06-15T12:00:00Z'::timestamptz + g * interval '1 minute'), 'row-' || g::text, 'item-' || g::text, g::float8 * 1.5, g::float8 * 2.5, NULL::int4, NULL::text, NULL::float8, NULL::date FROM generate_series(1,?) g" (PGSimple.Only n)
+                  let res :: Stream (Of BenchRow) (ResourceT IO) () = StreamingPostgresSimple.query pgSimpleConn sql17Simple (PGSimple.Only n)
                   S.effects res
         it ("postgresql-simple Record fold (" ++ show n ++ " rows)") $
           void $
diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index d767f62..276b473 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -800,6 +800,7 @@ instance FromPgField () where
       }
 
 {-# INLINE intRowDecoder #-}
+intRowDecoder :: RowDecoder Int
 intRowDecoder =
   inlinableRowDecoder haskellIntOids $ do
     fieldLen <- Parser.takeInt32BE
@@ -860,6 +861,17 @@ instance FromPgField Int16 where
         allowedPgTypes = (== int2Oid) . fieldTypeOid
       }
 
+{-# INLINE int32RowDecoder #-}
+int32RowDecoder :: RowDecoder Int32
+int32RowDecoder =
+  inlinableRowDecoder [int2Oid, int4Oid] $ do
+    fieldLen <- Parser.takeInt32BE
+    case fieldLen of
+      4 -> Parser.takeInt32BE
+      (-1) -> fail "Cannot decode SQL null as the Haskell Int32 type. Use a `Maybe Int32`"
+      2 -> fromIntegral <$> Parser.takeInt16BE
+      _ -> fail "Trying to decode PG int4 but it's not 2 or 4 bytes long"
+
 instance FromPgField Int32 where
   fieldDecoder =
     FieldDecoder
@@ -870,6 +882,22 @@ instance FromPgField Int32 where
                 Nothing -> Left "Cannot decode SQL null as the Haskell Int32 type. Use a `Maybe Int32`",
         allowedPgTypes = (`elem` [int2Oid, int4Oid]) . fieldTypeOid
       }
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = int32RowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = int32RowDecoder
+
+{-# INLINE int64RowDecoder #-}
+int64RowDecoder :: RowDecoder Int64
+int64RowDecoder =
+  inlinableRowDecoder [int2Oid, int4Oid, int8Oid] $ do
+    fieldLen <- Parser.takeInt32BE
+    case fieldLen of
+      8 -> Parser.takeInt64BE
+      4 -> fromIntegral <$> Parser.takeInt32BE
+      (-1) -> fail "Cannot decode SQL null as the Haskell Int64 type. Use a `Maybe Int64`"
+      2 -> fromIntegral <$> Parser.takeInt16BE
+      _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
 
 instance FromPgField Int64 where
   fieldDecoder =
@@ -881,6 +909,10 @@ instance FromPgField Int64 where
                 Nothing -> Left "Cannot decode SQL null as the Haskell Int64 type. Use a `Maybe Int64`",
         allowedPgTypes = (`elem` [int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = int64RowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = int64RowDecoder
 
 instance FromPgField Integer where
   fieldDecoder =
@@ -909,12 +941,25 @@ instance FromPgField Oid where
         allowedPgTypes = (== oidOid) . fieldTypeOid
       }
 
+{-# INLINE floatRowDecoder #-}
+floatRowDecoder :: RowDecoder Float
+floatRowDecoder =
+  let fromNullable = \case
+        Nothing -> fail "Cannot decode SQL null as the Haskell Float type. Use a `Maybe Float`"
+        Just i -> pure i
+   in inlinableRowDecoder [float4Oid] $ Parser.takeFloatBEWithFieldLength >>= fromNullable
+
 instance FromPgField Float where
   fieldDecoder = parsePgType [float4Oid] $ \case
     Just bs -> Right $ binaryFloat4Decoder bs
     Nothing -> Left "Cannot decode SQL null as the Haskell Float type. Use a `Maybe Float`"
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = floatRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = floatRowDecoder
 
 {-# INLINE doubleRowDecoder #-}
+doubleRowDecoder :: RowDecoder Double
 doubleRowDecoder =
   let fromNullable = \case
         Nothing -> fail "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`"
@@ -967,6 +1012,7 @@ typeMustBeNamed :: Text -> (FieldInfo -> Bool)
 typeMustBeNamed typName = \fieldInfo ->
   (typeName <$> lookupTypeByOid fieldInfo.fieldTypeOid fieldInfo.encodingContext.typeInfoCache) == Just typName
 
+{-# INLINE scientificDecoder #-}
 scientificDecoder :: Bool -> Parser.Parser Scientific
 scientificDecoder mustBeInteger = do
   ndigits <- Parser.takeInt16BE
@@ -984,24 +1030,50 @@ scientificDecoder mustBeInteger = do
       !digit <- fromIntegral <$> Parser.takeInt16BE
       parseAndMult (ndigitsLeft - 1) (currexpon - 4) (val + scientific digit currexpon)
 
+{-# INLINE numericRowParser #-}
+numericRowParser :: Parser.Parser Scientific
+numericRowParser = do
+  fieldLen <- Parser.takeInt32BE
+  case fieldLen of
+    (-1) -> fail "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`"
+    _ -> scientificDecoder False
+
 instance FromPgField Scientific where
   -- See https://github.com/postgres/postgres/blob/799959dc7cf0e2462601bea8d07b6edec3fa0c4f/src/backend/utils/adt/numeric.c#L1163
   fieldDecoder =
     FieldDecoder
-      { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
-          let !decodeInt = binaryIntDecoder @Int64 oid
-           in \case
-                Just bs ->
-                  -- TODO: There is loss converting from Float/Double to Scientific, but it might be quite small, so should we accept
-                  -- float4Oid and float8Oid here?
-                  if oid == numericOid
-                    then case Parser.parseOnly (scientificDecoder False <* Parser.endOfInput) bs of
-                      Parser.ParseOk sci -> Right sci
-                      Parser.ParseFail err -> Left err
-                    else flip scientific 0 . fromIntegral <$> decodeInt bs
-                Nothing -> Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`",
+      { fieldValueDecoder = \FieldInfo {fieldTypeOid} ->
+          if fieldTypeOid /= numericOid
+            then
+              let intdec = binaryIntDecoder @Int64 fieldTypeOid
+               in \case
+                    Just bs -> flip scientific 0 . fromIntegral <$> intdec bs
+                    Nothing -> Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`"
+            else \case
+              Just bs ->
+                -- TODO: There is loss converting from Float/Double to Scientific, but it might be quite small, so should we accept
+                -- float4Oid and float8Oid here?
+                case Parser.parseOnly (scientificDecoder False <* Parser.endOfInput) bs of
+                  Parser.ParseOk sci -> Right sci
+                  Parser.ParseFail err -> Left err
+              Nothing -> Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`",
         allowedPgTypes = (`elem` [numericOid, int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder =
+    RowDecoder
+      { fullRowDecoder = \case
+          [singleColInfo] ->
+            if singleColInfo.fieldTypeOid /= numericOid
+              then
+                flip scientific 0 . fromIntegral <$> (inlinedSingleFieldRowDecoder @Int64).fullRowDecoder [singleColInfo]
+              else numericRowParser
+          _ -> error "singleField expected a single column OID but got 0 or >1",
+        rowColumnsTypeCheck = \case
+          [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` [numericOid, int2Oid, int4Oid, int8Oid])]
+          _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+        numExpectedColumns = 1
+      }
 
 instance FromPgField (Ratio Integer) where
   fieldDecoder = toRational <$> fieldDecoder @Scientific
@@ -1009,15 +1081,22 @@ instance FromPgField (Ratio Integer) where
 binaryTrue :: ByteString
 binaryTrue = BinSer.encodePgBoolean True
 
+{-# INLINE boolRowDecoder #-}
+boolRowDecoder :: RowDecoder Bool
+boolRowDecoder =
+  let word8ToBool = \case
+        Nothing -> fail "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
+        Just w8 -> pure $ w8 == 1
+   in inlinableRowDecoder [boolOid] $ Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1 >>= word8ToBool
+
 instance FromPgField Bool where
   fieldDecoder = parsePgType [boolOid] $ \case
     Just bs -> Right $ bs == binaryTrue
     Nothing -> Left "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
-  singleFieldRowDecoder =
-    let word8ToBool = \case
-          Nothing -> fail "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
-          Just w8 -> pure $ w8 == 1
-     in inlinableRowDecoder [boolOid] $ Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1 >>= word8ToBool
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = boolRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = boolRowDecoder
 
 instance FromPgField Char where
   fieldDecoder =
@@ -1050,6 +1129,7 @@ instance FromPgField LBS.ByteString where
     Nothing -> Left "Cannot decode SQL null as the Haskell ByteString type. Use a `Maybe ByteString`"
 
 {-# INLINE textDecoder #-}
+textDecoder :: RowDecoder Text
 textDecoder =
   let fromNullable = \case
         Nothing -> fail "Cannot decode SQL null as the Haskell Text type. Use a `Maybe Text`"
@@ -1101,6 +1181,7 @@ instance FromPgField (CI String) where
   fieldDecoder = typeFieldDecoder (typeMustBeNamed "citext") $ CI.mk <$> fieldDecoder
 
 {-# INLINE utcTimeRowDecoder #-}
+utcTimeRowDecoder :: RowDecoder UTCTime
 utcTimeRowDecoder =
   let fromNullable = \case
         Nothing -> fail "Cannot decode SQL null as the Haskell UTCTime type. Use a `Maybe UTCTime`"
@@ -1194,6 +1275,7 @@ instance FromPgField TimeOfDay where
     Nothing -> Left "Cannot decode SQL null as the Haskell TimeOfDay type. Use a `Maybe TimeOfDay`"
 
 {-# INLINE dayRowDecoder #-}
+dayRowDecoder :: RowDecoder Day
 dayRowDecoder =
   let int32ToDay = \case
         Nothing -> fail "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
diff --git a/hpgsql/src/Hpgsql/SimpleParser.hs b/hpgsql/src/Hpgsql/SimpleParser.hs
index 23942e4..145c54d 100644
--- a/hpgsql/src/Hpgsql/SimpleParser.hs
+++ b/hpgsql/src/Hpgsql/SimpleParser.hs
@@ -30,6 +30,7 @@ module Hpgsql.SimpleParser
     takeInt16BEWithFieldLength,
     takeFloatBE,
     takeDoubleBE,
+    takeFloatBEWithFieldLength,
   )
 where
 
@@ -38,7 +39,7 @@ import Data.ByteString (ByteString)
 import qualified Data.ByteString as BS
 import Data.Int (Int16, Int32, Int64)
 import Foreign.Storable (Storable)
-import GHC.Float (castWord32ToFloat, castWord64ToDouble)
+import GHC.Float (castWord32ToFloat, castWord64ToDouble, word2Float)
 import Hpgsql.Encoding.BinarySerializer (ByteStringIdx (..))
 import qualified Hpgsql.Encoding.BinarySerializer as BinSer
 import Prelude hiding (take)
@@ -160,6 +161,15 @@ takeInt32BEWithFieldLength = do
   mi32 <- parsePgFieldWithAtMost4Bytes BinSer.TypeSize4
   pure $ fromIntegral <$> mi32
 
+{-# INLINE takeFloatBEWithFieldLength #-}
+
+-- | Parses both a field length and the field itself, for
+-- a Float in a row.
+takeFloatBEWithFieldLength :: Parser (Maybe Float)
+takeFloatBEWithFieldLength = do
+  mf <- parsePgFieldWithAtMost4Bytes BinSer.TypeSize4
+  pure $ castWord32ToFloat <$> mf
+
 {-# INLINE takeFloatBE #-}
 takeFloatBE :: Parser Float
 takeFloatBE = Parser $ \idx bs kf ks ->

From 53b339c5b31bcb074392192a3ef18a35f9dbf228 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Fri, 14 Aug 2026 14:19:48 -0300
Subject: [PATCH 09/22] Tests for non-specialized field decoders

---
 hpgsql-tests/EncodingDecodingSpec.hs | 518 ++++++++++++++++-----------
 1 file changed, 306 insertions(+), 212 deletions(-)

diff --git a/hpgsql-tests/EncodingDecodingSpec.hs b/hpgsql-tests/EncodingDecodingSpec.hs
index efdbdfc..d49bc84 100644
--- a/hpgsql-tests/EncodingDecodingSpec.hs
+++ b/hpgsql-tests/EncodingDecodingSpec.hs
@@ -372,8 +372,18 @@ byteaTextDecoding conn = hedgehog $ do
   someBs :: ByteString <- Gen.forAll $ Gen.bytes (Gen.linear 0 50)
   let lazyBs :: LBS.ByteString = LBS.fromStrict someBs
       hexStr = concatMap (\w -> let s = showHex w "" in if length s < 2 then '0' : s else s) (BS.unpack someBs)
-  res <- liftIO $ queryMay conn (fromString $ "SELECT '\\x" <> hexStr <> "'::bytea, '\\x" <> hexStr <> "'::bytea")
-  res === Just (someBs, lazyBs)
+      qry = fromString $ "SELECT '\\x" <> hexStr <> "'::bytea, '\\x" <> hexStr <> "'::bytea"
+  (res1, res2) <-
+    liftIO $
+      runPipeline conn $
+        (,)
+          <$> pipeline1With rowDecoder qry
+          -- Specialized row parsers of each type are a different implementation from
+          -- the simpler fieldDecoders, so we need to test both
+          <*> pipeline1With ((,) <$> singleField fieldDecoder <*> singleField fieldDecoder) qry
+  let expectedResult = (someBs, lazyBs)
+  liftIO res1 >>= (=== expectedResult)
+  liftIO res2 >>= (=== expectedResult)
 
 dateAndTimestampTextDecoding :: HPgConnection -> PropertyT IO ()
 dateAndTimestampTextDecoding conn = hedgehog $ do
@@ -393,38 +403,43 @@ dateAndTimestampTextDecoding conn = hedgehog $ do
       someNominalDiffTime :: NominalDiffTime = realToFrac $ picosecondsToDiffTime (someNominalDiffTimeMicros * 1_000_000)
       (intervalSecs, intervalRemMicros) = someIntervalTimeMicros `quotRem` 1_000_000
       (nomSecs, nomRemMicros) = someNominalDiffTimeMicros `quotRem` 1_000_000
-  res <-
+      qry =
+        fromString $
+          "SELECT '"
+            <> iso8601Show date
+            <> "'::date"
+            <> ", '"
+            <> iso8601Show timetz
+            <> "'::timestamptz"
+            <> ", '"
+            <> show someNumberOfMonths
+            <> " months "
+            <> show intervalSecs
+            <> " seconds "
+            <> show intervalRemMicros
+            <> " microseconds'::interval"
+            <> ", '"
+            <> iso8601Show timetz
+            <> "'::timestamptz"
+            <> ", '"
+            <> iso8601Show date
+            <> "'::date"
+            <> ", '"
+            <> show nomSecs
+            <> " seconds "
+            <> show nomRemMicros
+            <> " microseconds'::interval"
+  (res1, res2) <-
     liftIO $
-      queryWith
-        rowDecoder
-        conn
-        ( fromString $
-            "SELECT '"
-              <> iso8601Show date
-              <> "'::date"
-              <> ", '"
-              <> iso8601Show timetz
-              <> "'::timestamptz"
-              <> ", '"
-              <> show someNumberOfMonths
-              <> " months "
-              <> show intervalSecs
-              <> " seconds "
-              <> show intervalRemMicros
-              <> " microseconds'::interval"
-              <> ", '"
-              <> iso8601Show timetz
-              <> "'::timestamptz"
-              <> ", '"
-              <> iso8601Show date
-              <> "'::date"
-              <> ", '"
-              <> show nomSecs
-              <> " seconds "
-              <> show nomRemMicros
-              <> " microseconds'::interval"
-        )
-  res === [(date, timetz, someCalendarDiffTime, Finite timetz, Finite date, CalendarDiffTime 0 someNominalDiffTime)]
+      runPipeline conn $
+        (,)
+          <$> pipeline1With rowDecoder qry
+          -- Specialized row parsers of each type are a different implementation from
+          -- the simpler fieldDecoders, so we need to test both
+          <*> pipeline1With ((,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder) qry
+  let expectedResult = (date, timetz, someCalendarDiffTime, Finite timetz, Finite date, CalendarDiffTime 0 someNominalDiffTime)
+  liftIO res1 >>= (=== expectedResult)
+  liftIO res2 >>= (=== expectedResult)
 
 numericTextDecoding :: HPgConnection -> PropertyT IO ()
 numericTextDecoding conn = hedgehog $ do
@@ -433,30 +448,35 @@ numericTextDecoding conn = hedgehog $ do
   doubleVal :: Double <- Gen.forAll $ Gen.double $ Gen.exponentialFloatFrom 0 (-1e308) 1e308
   doubleVal2 :: Double <- Gen.forAll $ Gen.double $ Gen.linearFracFrom 0 (-1e308) 1e308
   integerVal :: Integer <- Gen.forAll $ (*) <$> (fromIntegral @Int64 <$> Gen.enumBounded) <*> (fromIntegral @Int64 <$> Gen.enumBounded)
-  res <-
+  let qry =
+        fromString $
+          "SELECT '1.521'::numeric, '1.521'::numeric(4,1), '1.521'::numeric"
+            <> ", '"
+            <> show floatVal
+            <> "'::float4"
+            <> ", '"
+            <> show floatVal2
+            <> "'::float4"
+            <> ", '"
+            <> show doubleVal
+            <> "'::float8"
+            <> ", '"
+            <> show doubleVal2
+            <> "'::float8"
+            <> ", '"
+            <> show integerVal
+            <> "'::numeric"
+  (res1, res2) <-
     liftIO $
-      queryWith
-        rowDecoder
-        conn
-        ( fromString $
-            "SELECT '1.521'::numeric, '1.521'::numeric(4,1), '1.521'::numeric"
-              <> ", '"
-              <> show floatVal
-              <> "'::float4"
-              <> ", '"
-              <> show floatVal2
-              <> "'::float4"
-              <> ", '"
-              <> show doubleVal
-              <> "'::float8"
-              <> ", '"
-              <> show doubleVal2
-              <> "'::float8"
-              <> ", '"
-              <> show integerVal
-              <> "'::numeric"
-        )
-  res === [(1.521 :: Scientific, 1.5 :: Scientific, 1.521 :: Scientific, floatVal, floatVal2, doubleVal, doubleVal2, integerVal)]
+      runPipeline conn $
+        (,)
+          <$> pipeline1With rowDecoder qry
+          -- Specialized row parsers of each type are a different implementation from
+          -- the simpler fieldDecoders, so we need to test both
+          <*> pipeline1With ((,,,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder) qry
+  let expectedResult = (1.521 :: Scientific, 1.5 :: Scientific, 1.521 :: Scientific, floatVal, floatVal2, doubleVal, doubleVal2, integerVal)
+  liftIO res1 >>= (=== expectedResult)
+  liftIO res2 >>= (=== expectedResult)
 
 numericTextDecodingLargerTypes :: HPgConnection -> PropertyT IO ()
 numericTextDecodingLargerTypes conn = hedgehog $ do
@@ -464,63 +484,107 @@ numericTextDecodingLargerTypes conn = hedgehog $ do
   int2Val :: Int16 <- Gen.forAll Gen.enumBounded
   int4Val :: Int32 <- Gen.forAll Gen.enumBounded
   int8Val :: Int64 <- Gen.forAll Gen.enumBounded
-  res <-
+  let qry =
+        fromString $
+          "SELECT '"
+            <> show floatVal
+            <> "'::float4"
+            <> ", '"
+            <> show int2Val
+            <> "'::int2"
+            <> ", '"
+            <> show int2Val
+            <> "'::int2"
+            <> ", '"
+            <> show int2Val
+            <> "'::int2"
+            <> ", '"
+            <> show int2Val
+            <> "'::int2"
+            <> ", '"
+            <> show int4Val
+            <> "'::int4"
+            <> ", '"
+            <> show int4Val
+            <> "'::int4"
+            <> ", '"
+            <> show int4Val
+            <> "'::int4"
+            <> ", '"
+            <> show int8Val
+            <> "'::int8"
+            <> ", '"
+            <> show int8Val
+            <> "'::int8"
+  (res1, res2) <-
     liftIO $
-      queryWith
-        rowDecoder
-        conn
-        ( fromString $
-            "SELECT '"
-              <> show floatVal
-              <> "'::float4"
-              <> ", '"
-              <> show int2Val
-              <> "'::int2"
-              <> ", '"
-              <> show int2Val
-              <> "'::int2"
-              <> ", '"
-              <> show int2Val
-              <> "'::int2"
-              <> ", '"
-              <> show int2Val
-              <> "'::int2"
-              <> ", '"
-              <> show int4Val
-              <> "'::int4"
-              <> ", '"
-              <> show int4Val
-              <> "'::int4"
-              <> ", '"
-              <> show int4Val
-              <> "'::int4"
-              <> ", '"
-              <> show int8Val
-              <> "'::int8"
-              <> ", '"
-              <> show int8Val
-              <> "'::int8"
-        )
-  let rowRes = (float2Double floatVal, fromIntegral int2Val :: Int32, fromIntegral int2Val :: Int64, fromIntegral int2Val :: Integer, fromIntegral int2Val :: Scientific, fromIntegral int4Val :: Int64, fromIntegral int4Val :: Integer, fromIntegral int4Val :: Scientific, fromIntegral int8Val :: Integer, fromIntegral int8Val :: Scientific)
-  res === [rowRes]
+      runPipeline conn $
+        (,)
+          <$> pipeline1With rowDecoder qry
+          -- Specialized row parsers of each type are a different implementation from
+          -- the simpler fieldDecoders, so we need to test both
+          <*> pipeline1With ((,,,,,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder) qry
+  let expectedResult = (float2Double floatVal, fromIntegral int2Val :: Int32, fromIntegral int2Val :: Int64, fromIntegral int2Val :: Integer, fromIntegral int2Val :: Scientific, fromIntegral int4Val :: Int64, fromIntegral int4Val :: Integer, fromIntegral int4Val :: Scientific, fromIntegral int8Val :: Integer, fromIntegral int8Val :: Scientific)
+  liftIO res1 >>= (=== expectedResult)
+  liftIO res2 >>= (=== expectedResult)
 
 numericExtremeTextDecoding :: HPgConnection -> IO ()
 numericExtremeTextDecoding conn = do
-  queryWith rowDecoder conn (fromString $ "SELECT '" <> show (minBound :: Int16) <> "'::int2, '" <> show (maxBound :: Int16) <> "'::int2")
-    `shouldReturn` [(minBound :: Int16, maxBound :: Int16)]
-  queryWith rowDecoder conn (fromString $ "SELECT '" <> show (minBound :: Int32) <> "'::int4, '" <> show (maxBound :: Int32) <> "'::int4")
-    `shouldReturn` [(minBound :: Int32, maxBound :: Int32)]
-  queryWith rowDecoder conn (fromString $ "SELECT '" <> show (minBound :: Int64) <> "'::int8, '" <> show (maxBound :: Int64) <> "'::int8")
-    `shouldReturn` [(minBound :: Int64, maxBound :: Int64)]
-  [(f :: Float, d :: Double)] <- queryWith rowDecoder conn "SELECT 'NaN'::float4, 'NaN'::float8"
-  f `shouldSatisfy` isNaN
-  d `shouldSatisfy` isNaN
-  queryWith rowDecoder conn "SELECT 'Infinity'::float4, '-Infinity'::float4, 'Infinity'::float8, '-Infinity'::float8"
-    `shouldReturn` [((1 / 0) :: Float, ((-1) / 0) :: Float, (1 / 0) :: Double, ((-1) / 0) :: Double)]
-  [(d1 :: Double, d2 :: Double, d3 :: Double)] <- queryWith rowDecoder conn "SELECT 'NaN'::float4, 'Infinity'::float4, '-Infinity'::float4"
+  -- Specialized row parsers of each type are a different implementation from
+  -- the simpler fieldDecoders, so we need to test both
+  let int16Qry = fromString $ "SELECT '" <> show (minBound :: Int16) <> "'::int2, '" <> show (maxBound :: Int16) <> "'::int2"
+      int32Qry = fromString $ "SELECT '" <> show (minBound :: Int32) <> "'::int4, '" <> show (maxBound :: Int32) <> "'::int4"
+      int64Qry = fromString $ "SELECT '" <> show (minBound :: Int64) <> "'::int8, '" <> show (maxBound :: Int64) <> "'::int8"
+      nanQry = "SELECT 'NaN'::float4, 'NaN'::float8"
+      infQry = "SELECT 'Infinity'::float4, '-Infinity'::float4, 'Infinity'::float8, '-Infinity'::float8"
+      mixQry = "SELECT 'NaN'::float4, 'Infinity'::float4, '-Infinity'::float4"
+  (int16Res1, int16Res2, int32Res1, int32Res2, int64Res1, int64Res2, nanRes1, nanRes2, infRes1, infRes2, mixRes1, mixRes2) <-
+    runPipeline conn $
+      (,,,,,,,,,,,)
+        <$> pipeline1With rowDecoder int16Qry
+        <*> pipeline1With ((,) <$> singleField fieldDecoder <*> singleField fieldDecoder) int16Qry
+        <*> pipeline1With rowDecoder int32Qry
+        <*> pipeline1With ((,) <$> singleField fieldDecoder <*> singleField fieldDecoder) int32Qry
+        <*> pipeline1With rowDecoder int64Qry
+        <*> pipeline1With ((,) <$> singleField fieldDecoder <*> singleField fieldDecoder) int64Qry
+        <*> pipeline1With rowDecoder nanQry
+        <*> pipeline1With ((,) <$> singleField fieldDecoder <*> singleField fieldDecoder) nanQry
+        <*> pipeline1With rowDecoder infQry
+        <*> pipeline1With ((,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder) infQry
+        <*> pipeline1With rowDecoder mixQry
+        <*> pipeline1With ((,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder) mixQry
+  -- Integer boundary values
+  int16Res1 `shouldReturn` (minBound :: Int16, maxBound :: Int16)
+  int16Res2 `shouldReturn` (minBound :: Int16, maxBound :: Int16)
+  int32Res1 `shouldReturn` (minBound :: Int32, maxBound :: Int32)
+  int32Res2 `shouldReturn` (minBound :: Int32, maxBound :: Int32)
+  int64Res1 `shouldReturn` (minBound :: Int64, maxBound :: Int64)
+  int64Res2 `shouldReturn` (minBound :: Int64, maxBound :: Int64)
+  -- NaN for Float and Double
+  (f1 :: Float, d1 :: Double) <- nanRes1
+  f1 `shouldSatisfy` isNaN
   d1 `shouldSatisfy` isNaN
-  d2 `shouldBe` (1 / 0 :: Double)
-  d3 `shouldBe` ((-1) / 0 :: Double)
+  (f2 :: Float, d2 :: Double) <- nanRes2
+  f2 `shouldSatisfy` isNaN
+  d2 `shouldSatisfy` isNaN
+  -- +-Infinity for Float and Double
+  let infRow = (posInfFloat, negInfFloat, posInfDouble, negInfDouble)
+  infRes1 `shouldReturn` infRow
+  infRes2 `shouldReturn` infRow
+  -- NaN and +-Infinity encoded as Float, decoded as Double
+  (md1 :: Double, md2 :: Double, md3 :: Double) <- mixRes1
+  md1 `shouldSatisfy` isNaN
+  md2 `shouldBe` posInfDouble
+  md3 `shouldBe` negInfDouble
+  (md4 :: Double, md5 :: Double, md6 :: Double) <- mixRes2
+  md4 `shouldSatisfy` isNaN
+  md5 `shouldBe` posInfDouble
+  md6 `shouldBe` negInfDouble
+  where
+    posInfFloat = (1 / 0) :: Float
+    negInfFloat = ((-1) / 0) :: Float
+    posInfDouble = (1 / 0) :: Double
+    negInfDouble = ((-1) / 0) :: Double
 
 jsonTextDecoding :: HPgConnection -> PropertyT IO ()
 jsonTextDecoding conn = hedgehog $ do
@@ -528,29 +592,38 @@ jsonTextDecoding conn = hedgehog $ do
   jsonVal2 :: Aeson.Value <- Gen.forAll genJsonValue
   jsonVal3 :: Aeson.Value <- Gen.forAll genJsonValue
   let encodeJson = pgEscape . Text.unpack . TE.decodeUtf8 . LBS.toStrict . Aeson.encode
-  [(v1, v2, v3, v4) :: (Aeson.Value, Aeson.Value, PgJson, PgJson)] <-
+      qry =
+        fromString $
+          "SELECT '"
+            <> encodeJson jsonVal1
+            <> "'::json"
+            <> ", '"
+            <> encodeJson jsonVal1
+            <> "'::jsonb"
+            <> ", '"
+            <> encodeJson jsonVal2
+            <> "'::json"
+            <> ", '"
+            <> encodeJson jsonVal3
+            <> "'::jsonb"
+  -- Specialized row parsers of each type are a different implementation from
+  -- the simpler fieldDecoders, so we need to test both
+  (res1, res2) <-
     liftIO $
-      queryWith
-        rowDecoder
-        conn
-        ( fromString $
-            "SELECT '"
-              <> encodeJson jsonVal1
-              <> "'::json"
-              <> ", '"
-              <> encodeJson jsonVal1
-              <> "'::jsonb"
-              <> ", '"
-              <> encodeJson jsonVal2
-              <> "'::json"
-              <> ", '"
-              <> encodeJson jsonVal3
-              <> "'::jsonb"
-        )
+      runPipeline conn $
+        (,)
+          <$> pipeline1With rowDecoder qry
+          <*> pipeline1With ((,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder) qry
+  (v1, v2, v3, v4) :: (Aeson.Value, Aeson.Value, PgJson, PgJson) <- liftIO res1
   v1 === jsonVal1
   v2 === jsonVal1
   Aeson.toJSON v3 === jsonVal2
   Aeson.toJSON v4 === jsonVal3
+  (v5, v6, v7, v8) :: (Aeson.Value, Aeson.Value, PgJson, PgJson) <- liftIO res2
+  v5 === jsonVal1
+  v6 === jsonVal1
+  Aeson.toJSON v7 === jsonVal2
+  Aeson.toJSON v8 === jsonVal3
   where
     pgEscape = concatMap $ \case
       '\'' -> "''"
@@ -570,13 +643,18 @@ uuidTextDecoding :: HPgConnection -> PropertyT IO ()
 uuidTextDecoding conn = hedgehog $ do
   uuidBytes <- Gen.forAll $ Gen.bytes (Gen.singleton 16)
   let Just uuid = UUID.fromByteString (LBS.fromStrict uuidBytes)
-  res <-
+      qry = fromString $ "SELECT '" <> UUID.toString uuid <> "'::uuid"
+  (res1, res2) <-
     liftIO $
-      queryWith
-        rowDecoder
-        conn
-        (fromString $ "SELECT '" <> UUID.toString uuid <> "'::uuid")
-  res === [Only uuid]
+      runPipeline conn $
+        (,)
+          <$> pipeline1With rowDecoder qry
+          -- Specialized row parsers of each type are a different implementation from
+          -- the simpler fieldDecoders, so we need to test both
+          <*> pipeline1With (Only <$> singleField fieldDecoder) qry
+  let expectedResult = Only uuid
+  liftIO res1 >>= (=== expectedResult)
+  liftIO res2 >>= (=== expectedResult)
 
 ciTextRoundTrip :: HPgConnection -> PropertyT IO ()
 ciTextRoundTrip conn = hedgehog $ do
@@ -604,13 +682,18 @@ ciTextRoundTrip conn = hedgehog $ do
 ciTextTextDecoding :: HPgConnection -> PropertyT IO ()
 ciTextTextDecoding conn = hedgehog $ do
   someText :: Text <- Gen.forAll $ Gen.text (Gen.linear 0 50) (Gen.filter (\c -> c /= '\0' && c /= '\'') Gen.unicode)
-  res <-
-    liftIO $ do
-      queryWith
-        rowDecoder
-        conn
-        (fromString $ "SELECT '" <> Text.unpack someText <> "'::citext, '" <> Text.unpack someText <> "'::citext, '" <> Text.unpack someText <> "'::citext")
-  res === [(CI.mk someText, CI.mk (LT.fromStrict someText), CI.mk (Text.unpack someText))]
+  let qry = fromString $ "SELECT '" <> Text.unpack someText <> "'::citext, '" <> Text.unpack someText <> "'::citext, '" <> Text.unpack someText <> "'::citext"
+  (res1, res2) <-
+    liftIO $
+      runPipeline conn $
+        (,)
+          <$> pipeline1With rowDecoder qry
+          -- Specialized row parsers of each type are a different implementation from
+          -- the simpler fieldDecoders, so we need to test both
+          <*> pipeline1With ((,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder) qry
+  let expectedResult = (CI.mk someText, CI.mk (LT.fromStrict someText), CI.mk (Text.unpack someText))
+  liftIO res1 >>= (=== expectedResult)
+  liftIO res2 >>= (=== expectedResult)
 
 timeOfDayRoundTrip :: HPgConnection -> PropertyT IO ()
 timeOfDayRoundTrip conn = hedgehog $ do
@@ -642,43 +725,48 @@ timeOfDayTextDecoding conn = hedgehog $ do
         pure $ timeToTimeOfDay $ picosecondsToDiffTime (timeOfDayMicros * 1_000_000)
   row <- Gen.forAll $ (,,,,,,,,,) <$> genTimeOfDay <*> genTimeOfDay <*> genTimeOfDay <*> genTimeOfDay <*> genTimeOfDay <*> genTimeOfDay <*> genTimeOfDay <*> genTimeOfDay <*> genTimeOfDay <*> genTimeOfDay
   let (t1, t2, t3, t4, t5, t6, t7, t8, t9, t10) = row
-  res <-
+      qry =
+        fromString $
+          "SELECT '"
+            <> iso8601Show t1
+            <> "'::time"
+            <> ", '"
+            <> iso8601Show t2
+            <> "'::time"
+            <> ", '"
+            <> iso8601Show t3
+            <> "'::time"
+            <> ", '"
+            <> iso8601Show t4
+            <> "'::time"
+            <> ", '"
+            <> iso8601Show t5
+            <> "'::time"
+            <> ", '"
+            <> iso8601Show t6
+            <> "'::time"
+            <> ", '"
+            <> iso8601Show t7
+            <> "'::time"
+            <> ", '"
+            <> iso8601Show t8
+            <> "'::time"
+            <> ", '"
+            <> iso8601Show t9
+            <> "'::time"
+            <> ", '"
+            <> iso8601Show t10
+            <> "'::time"
+  (res1, res2) <-
     liftIO $
-      query
-        conn
-        ( fromString $
-            "SELECT '"
-              <> iso8601Show t1
-              <> "'::time"
-              <> ", '"
-              <> iso8601Show t2
-              <> "'::time"
-              <> ", '"
-              <> iso8601Show t3
-              <> "'::time"
-              <> ", '"
-              <> iso8601Show t4
-              <> "'::time"
-              <> ", '"
-              <> iso8601Show t5
-              <> "'::time"
-              <> ", '"
-              <> iso8601Show t6
-              <> "'::time"
-              <> ", '"
-              <> iso8601Show t7
-              <> "'::time"
-              <> ", '"
-              <> iso8601Show t8
-              <> "'::time"
-              <> ", '"
-              <> iso8601Show t9
-              <> "'::time"
-              <> ", '"
-              <> iso8601Show t10
-              <> "'::time"
-        )
-  res === [row]
+      runPipeline conn $
+        (,)
+          <$> pipeline1With rowDecoder qry
+          -- Specialized row parsers of each type are a different implementation from
+          -- the simpler fieldDecoders, so we need to test both
+          <*> pipeline1With ((,,,,,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder) qry
+  liftIO res1 >>= (=== row)
+  liftIO res2 >>= (=== row)
 
 localTimeTextDecoding :: HPgConnection -> PropertyT IO ()
 localTimeTextDecoding conn = hedgehog $ do
@@ -692,7 +780,39 @@ localTimeTextDecoding conn = hedgehog $ do
         pure $ LocalTime localDay localTimeOfDay
   row <- Gen.forAll $ (,,,,,,,,,) <$> genLocalTime <*> genLocalTime <*> genLocalTime <*> genLocalTime <*> genLocalTime <*> genLocalTime <*> genLocalTime <*> genLocalTime <*> genLocalTime <*> genLocalTime
   let (lt1, lt2, lt3, lt4, lt5, lt6, lt7, lt8, lt9, lt10) = row
-  res <-
+      qry =
+        fromString $
+          "SELECT '"
+            <> iso8601Show lt1
+            <> "'::timestamp"
+            <> ", '"
+            <> iso8601Show lt2
+            <> "'::timestamp"
+            <> ", '"
+            <> iso8601Show lt3
+            <> "'::timestamp"
+            <> ", '"
+            <> iso8601Show lt4
+            <> "'::timestamp"
+            <> ", '"
+            <> iso8601Show lt5
+            <> "'::timestamp"
+            <> ", '"
+            <> iso8601Show lt6
+            <> "'::timestamp"
+            <> ", '"
+            <> iso8601Show lt7
+            <> "'::timestamp"
+            <> ", '"
+            <> iso8601Show lt8
+            <> "'::timestamp"
+            <> ", '"
+            <> iso8601Show lt9
+            <> "'::timestamp"
+            <> ", '"
+            <> iso8601Show lt10
+            <> "'::timestamp"
+  (res1Val, res2Val) <-
     liftIO $ withRollback conn $ do
       -- Doesn't seem like the timezone matters, but we set to
       -- UTC because this is a textual representation, and the
@@ -701,42 +821,16 @@ localTimeTextDecoding conn = hedgehog $ do
       -- are the inverse of each other but produce bogus values
       -- nonetheless.
       execute conn "SET LOCAL timezone = 'UTC'"
-      queryWith
-        rowDecoder
-        conn
-        ( fromString $
-            "SELECT '"
-              <> iso8601Show lt1
-              <> "'::timestamp"
-              <> ", '"
-              <> iso8601Show lt2
-              <> "'::timestamp"
-              <> ", '"
-              <> iso8601Show lt3
-              <> "'::timestamp"
-              <> ", '"
-              <> iso8601Show lt4
-              <> "'::timestamp"
-              <> ", '"
-              <> iso8601Show lt5
-              <> "'::timestamp"
-              <> ", '"
-              <> iso8601Show lt6
-              <> "'::timestamp"
-              <> ", '"
-              <> iso8601Show lt7
-              <> "'::timestamp"
-              <> ", '"
-              <> iso8601Show lt8
-              <> "'::timestamp"
-              <> ", '"
-              <> iso8601Show lt9
-              <> "'::timestamp"
-              <> ", '"
-              <> iso8601Show lt10
-              <> "'::timestamp"
-        )
-  res === [row]
+      (res1, res2) <-
+        runPipeline conn $
+          (,)
+            <$> pipeline1With rowDecoder qry
+            -- Specialized row parsers of each type are a different implementation from
+            -- the simpler fieldDecoders, so we need to test both
+            <*> pipeline1With ((,,,,,,,,,) <$> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder <*> singleField fieldDecoder) qry
+      (,) <$> res1 <*> res2
+  res1Val === row
+  res2Val === row
 
 fieldDecoderSemigroup :: HPgConnection -> IO ()
 fieldDecoderSemigroup conn = do

From d46a807680390de5bf07f889f860925a97cf05b4 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Fri, 14 Aug 2026 14:22:37 -0300
Subject: [PATCH 10/22] Tidy up GHC Core

---
 hpgsql-benchmarks/src/Main.hs     | 2 +-
 hpgsql-tests/RowDecoderGhcCore.hs | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/hpgsql-benchmarks/src/Main.hs b/hpgsql-benchmarks/src/Main.hs
index 58420e4..b206ed1 100644
--- a/hpgsql-benchmarks/src/Main.hs
+++ b/hpgsql-benchmarks/src/Main.hs
@@ -1,4 +1,4 @@
-{-# OPTIONS_GHC -ddump-simpl -ddump-to-file #-}
+{-# OPTIONS_GHC -ddump-simpl -dno-typeable-binds -dsuppress-coercions -dsuppress-module-prefixes -dsuppress-type-applications -ddump-to-file #-}
 
 module Main where
 
diff --git a/hpgsql-tests/RowDecoderGhcCore.hs b/hpgsql-tests/RowDecoderGhcCore.hs
index 0e1fab9..244ec4d 100644
--- a/hpgsql-tests/RowDecoderGhcCore.hs
+++ b/hpgsql-tests/RowDecoderGhcCore.hs
@@ -1,4 +1,4 @@
-{-# OPTIONS_GHC -ddump-simpl -ddump-to-file #-}
+{-# OPTIONS_GHC -ddump-simpl -dno-typeable-binds -dsuppress-coercions -dsuppress-module-prefixes -dsuppress-type-applications -ddump-to-file #-}
 
 -- |
 -- This is not a real test module. It's just a type deriving `FromPgRow`

From ef7ce8d4fcb68e63038016fd1f928a360a4f5343 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Sat, 15 Aug 2026 09:52:40 -0300
Subject: [PATCH 11/22] Overlapping Maybe instances do help with the inlined
 row decoder's performance

---
 hpgsql-benchmarks/src/Main.hs |   4 +-
 hpgsql/src/Hpgsql/Encoding.hs | 211 ++++++++++++++++++++--------------
 2 files changed, 129 insertions(+), 86 deletions(-)

diff --git a/hpgsql-benchmarks/src/Main.hs b/hpgsql-benchmarks/src/Main.hs
index b206ed1..25672a8 100644
--- a/hpgsql-benchmarks/src/Main.hs
+++ b/hpgsql-benchmarks/src/Main.hs
@@ -86,8 +86,8 @@ data BenchRow = BenchRow
     brMaybeDay :: !(Maybe Day),
     brNumeric :: !Scientific,
     brFloat :: !Float,
-    brBool1 :: Bool,
-    brBool2 :: Bool
+    brBool1 :: !Bool,
+    brBool2 :: !Bool
   }
   deriving stock (Generic, Show, Eq)
   deriving anyclass (NFData, Hpgsql.FromPgRow, PGSimple.FromRow)
diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index 276b473..7f32c19 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -800,17 +800,17 @@ instance FromPgField () where
       }
 
 {-# INLINE intRowDecoder #-}
-intRowDecoder :: RowDecoder Int
+intRowDecoder :: RowDecoder (Maybe Int)
 intRowDecoder =
   inlinableRowDecoder haskellIntOids $ do
     fieldLen <- Parser.takeInt32BE
     -- TODO: We're assuming `Int` is always 64 bits, so 64bit CPUs? Is that ok?
     -- TODO: Is there a way to optimistically assume <=4 bytes and use our custom new parser?
     case fieldLen of
-      4 -> fromIntegral <$> Parser.takeInt32BE
-      (-1) -> fail "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`"
-      8 -> fromIntegral <$> Parser.takeInt64BE
-      2 -> fromIntegral <$> Parser.takeInt16BE
+      4 -> Just . fromIntegral <$> Parser.takeInt32BE
+      (-1) -> pure Nothing
+      8 -> Just . fromIntegral <$> Parser.takeInt64BE
+      2 -> Just . fromIntegral <$> Parser.takeInt16BE
       _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
 
 instance FromPgField Int where
@@ -824,32 +824,28 @@ instance FromPgField Int where
         allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = intRowDecoder
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = nonNullableRowDec "Int" intRowDecoder
+
+instance {-# OVERLAPPING #-} FromPgField (Maybe Int) where
+  -- This overlapping instance isn't pretty, but it reduces memory
+  -- usage and improves performance a bit
+  fieldDecoder = error "TODO Maybe Int"
+
+  -- FieldDecoder
+  --   { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
+  --       let !decode = binaryIntDecoder oid
+  --        in \case
+  --             Just bs -> Just <$> decode bs
+  --             Nothing -> Right Nothing,
+  --     allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
+  --   }
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = intRowDecoder
 
--- The instance below makes our Records benchmark faster and use less
--- memory, but makes our Tuples benchmark slower. Worth investigating.
--- instance {-# OVERLAPPING #-} FromPgField (Maybe Int) where
---   fieldDecoder = error "NOOO"
-
---   -- FieldDecoder
---   --   { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
---   --       let !decode = binaryIntDecoder oid
---   --        in \case
---   --             Just bs -> Just <$> decode bs
---   --             Nothing -> Right Nothing,
---   --     allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
---   --   }
---   singleFieldRowDecoder =
---     RowDecoder
---       { fullRowDecoder = const binaryIntSpecializedRowDecoder,
---         rowColumnsTypeCheck = \case
---           [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` haskellIntOids)]
---           _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
---         numExpectedColumns = 1
---       }
-
 instance FromPgField Int16 where
   fieldDecoder =
     FieldDecoder
@@ -942,35 +938,38 @@ instance FromPgField Oid where
       }
 
 {-# INLINE floatRowDecoder #-}
-floatRowDecoder :: RowDecoder Float
+floatRowDecoder :: RowDecoder (Maybe Float)
 floatRowDecoder =
-  let fromNullable = \case
-        Nothing -> fail "Cannot decode SQL null as the Haskell Float type. Use a `Maybe Float`"
-        Just i -> pure i
-   in inlinableRowDecoder [float4Oid] $ Parser.takeFloatBEWithFieldLength >>= fromNullable
+  inlinableRowDecoder [float4Oid] Parser.takeFloatBEWithFieldLength
 
 instance FromPgField Float where
   fieldDecoder = parsePgType [float4Oid] $ \case
     Just bs -> Right $ binaryFloat4Decoder bs
     Nothing -> Left "Cannot decode SQL null as the Haskell Float type. Use a `Maybe Float`"
   {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = floatRowDecoder
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = nonNullableRowDec "Float" floatRowDecoder
+
+instance {-# OVERLAPPING #-} FromPgField (Maybe Float) where
+  -- This overlapping instance isn't pretty, but it reduces memory
+  -- usage and improves performance a bit
+  fieldDecoder = error "TODO Maybe Float"
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = floatRowDecoder
 
 {-# INLINE doubleRowDecoder #-}
-doubleRowDecoder :: RowDecoder Double
+doubleRowDecoder :: RowDecoder (Maybe Double)
 doubleRowDecoder =
-  let fromNullable = \case
-        Nothing -> fail "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`"
-        Just i -> pure i
-      float4OrDouble8Decoder = do
+  let float4OrDouble8Decoder = do
         len <- Parser.takeInt32BE
         case len of
           8 -> Just <$> Parser.takeDoubleBE
           4 -> Just . float2Double <$> Parser.takeFloatBE
-          _ -> fail "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`"
-   in inlinableRowDecoder [float8Oid, float4Oid] $ float4OrDouble8Decoder >>= fromNullable
+          _ -> pure Nothing
+   in inlinableRowDecoder [float8Oid, float4Oid] float4OrDouble8Decoder
 
 instance FromPgField Double where
   fieldDecoder =
@@ -985,7 +984,16 @@ instance FromPgField Double where
         allowedPgTypes = (`elem` [float8Oid, float4Oid]) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = doubleRowDecoder
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = nonNullableRowDec "Double" doubleRowDecoder
+
+instance {-# OVERLAPPING #-} FromPgField (Maybe Double) where
+  -- This overlapping instance isn't pretty, but it reduces memory
+  -- usage and improves performance a bit
+  fieldDecoder = error "TODO Maybe Double"
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = doubleRowDecoder
 
@@ -1082,19 +1090,25 @@ binaryTrue :: ByteString
 binaryTrue = BinSer.encodePgBoolean True
 
 {-# INLINE boolRowDecoder #-}
-boolRowDecoder :: RowDecoder Bool
+boolRowDecoder :: RowDecoder (Maybe Bool)
 boolRowDecoder =
-  let word8ToBool = \case
-        Nothing -> fail "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
-        Just w8 -> pure $ w8 == 1
-   in inlinableRowDecoder [boolOid] $ Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1 >>= word8ToBool
+  inlinableRowDecoder [boolOid] $ fmap (== 1) <$> Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1
 
 instance FromPgField Bool where
   fieldDecoder = parsePgType [boolOid] $ \case
     Just bs -> Right $ bs == binaryTrue
     Nothing -> Left "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
   {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = boolRowDecoder
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = nonNullableRowDec "Bool" boolRowDecoder
+
+instance {-# OVERLAPPING #-} FromPgField (Maybe Bool) where
+  -- This overlapping instance isn't pretty, but it reduces memory
+  -- usage and improves performance a bit
+  fieldDecoder = error "TODO Maybe Bool"
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = boolRowDecoder
 
@@ -1129,18 +1143,14 @@ instance FromPgField LBS.ByteString where
     Nothing -> Left "Cannot decode SQL null as the Haskell ByteString type. Use a `Maybe ByteString`"
 
 {-# INLINE textDecoder #-}
-textDecoder :: RowDecoder Text
+textDecoder :: RowDecoder (Maybe Text)
 textDecoder =
-  let fromNullable = \case
-        Nothing -> fail "Cannot decode SQL null as the Haskell Text type. Use a `Maybe Text`"
-        Just i -> pure i
-      rp = do
-        len <- Parser.takeInt32BE
-        if len >= 0
-          -- TODO: Use some faster unsafeDecodeUtf8 function?
-          then Just . decodeUtf8 <$> Parser.take (fromIntegral len)
-          else pure Nothing
-   in inlinableRowDecoder [textOid, varcharOid, nameOid] $ rp >>= fromNullable
+  inlinableRowDecoder [textOid, varcharOid, nameOid] $ do
+    len <- Parser.takeInt32BE
+    if len >= 0
+      -- TODO: Use some faster unsafeDecodeUtf8 function?
+      then Just . decodeUtf8 <$> Parser.take (fromIntegral len)
+      else pure Nothing
 
 instance FromPgField Text where
   fieldDecoder = parsePgType [textOid, varcharOid, nameOid] $ \case
@@ -1148,7 +1158,16 @@ instance FromPgField Text where
     -- TODO: Use some faster unsafeDecodeUtf8 function?
     Nothing -> Left "Cannot decode SQL null as the Haskell Text type. Use a `Maybe Text`"
   {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = textDecoder
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = nonNullableRowDec "Text" textDecoder
+
+instance {-# OVERLAPPING #-} FromPgField (Maybe Text) where
+  -- This overlapping instance isn't pretty, but it reduces memory
+  -- usage and improves performance a bit
+  fieldDecoder = error "TODO Maybe Text"
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = textDecoder
 
@@ -1181,21 +1200,17 @@ instance FromPgField (CI String) where
   fieldDecoder = typeFieldDecoder (typeMustBeNamed "citext") $ CI.mk <$> fieldDecoder
 
 {-# INLINE utcTimeRowDecoder #-}
-utcTimeRowDecoder :: RowDecoder UTCTime
+utcTimeRowDecoder :: RowDecoder (Maybe UTCTime)
 utcTimeRowDecoder =
-  let fromNullable = \case
-        Nothing -> fail "Cannot decode SQL null as the Haskell UTCTime type. Use a `Maybe UTCTime`"
-        Just i -> pure i
-      utcTimeDecoder = do
-        len <- Parser.takeInt32BE
-        case len of
-          8 -> do
-            totalusecs <- Parser.takeInt64BE
-            let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
-                parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-            pure $ Just $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-          _ -> pure Nothing
-   in inlinableRowDecoder [timestamptzOid] $ utcTimeDecoder >>= fromNullable
+  inlinableRowDecoder [timestamptzOid] $ do
+    len <- Parser.takeInt32BE
+    case len of
+      8 -> do
+        totalusecs <- Parser.takeInt64BE
+        let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
+            parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
+        pure $ Just $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
+      _ -> pure Nothing
 
 instance FromPgField UTCTime where
   fieldDecoder = parsePgType [timestamptzOid] $ \case
@@ -1207,12 +1222,18 @@ instance FromPgField UTCTime where
       Right $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
     Nothing -> Left "Cannot decode SQL null as the Haskell UTCTime type. Use a `Maybe UTCTime`"
   {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = utcTimeRowDecoder
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# NOINLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = utcTimeRowDecoder
+  inlinedSingleFieldRowDecoder = nonNullableRowDec "UTCTime" utcTimeRowDecoder
 
--- {-# INLINE inlinedSingleFieldRowDecoder #-}
--- inlinedSingleFieldRowDecoder = doubleRowDecoder
+instance {-# OVERLAPPING #-} FromPgField (Maybe UTCTime) where
+  -- This overlapping instance isn't pretty, but it reduces memory
+  -- usage and improves performance a bit
+  fieldDecoder = error "TODO Maybe UTCTime"
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = utcTimeRowDecoder
 
 instance FromPgField (Unbounded UTCTime) where
   fieldDecoder = parsePgType [timestamptzOid] $ \case
@@ -1275,12 +1296,10 @@ instance FromPgField TimeOfDay where
     Nothing -> Left "Cannot decode SQL null as the Haskell TimeOfDay type. Use a `Maybe TimeOfDay`"
 
 {-# INLINE dayRowDecoder #-}
-dayRowDecoder :: RowDecoder Day
+dayRowDecoder :: RowDecoder (Maybe Day)
 dayRowDecoder =
-  let int32ToDay = \case
-        Nothing -> fail "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
-        Just i32 -> let jd = fromIntegral i32 :: Integer in pure $ addJulianDurationClip (CalendarDiffDays 0 (jd - 13)) $ fromJulian 2000 01 01
-   in inlinableRowDecoder [dateOid] $ Parser.takeInt32BEWithFieldLength >>= int32ToDay
+  let int32ToDay (i32 :: Int32) = let jd = fromIntegral i32 :: Integer in addJulianDurationClip (CalendarDiffDays 0 (jd - 13)) $ fromJulian 2000 01 01
+   in inlinableRowDecoder [dateOid] $ fmap int32ToDay <$> Parser.takeInt32BEWithFieldLength
 
 instance FromPgField Day where
   fieldDecoder = parsePgType [dateOid] $ \case
@@ -1292,7 +1311,16 @@ instance FromPgField Day where
       Right $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
     Nothing -> Left "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
   {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = dayRowDecoder
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = nonNullableRowDec "Day" dayRowDecoder
+
+instance {-# OVERLAPPING #-} FromPgField (Maybe Day) where
+  -- This overlapping instance isn't pretty, but it reduces memory
+  -- usage and improves performance a bit
+  fieldDecoder = error "TODO Maybe Day"
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = dayRowDecoder
 
@@ -1357,9 +1385,24 @@ nullableField FieldDecoder {..} =
       allowedPgTypes
     }
 
+{-# INLINE nonNullableRowDec #-}
+nonNullableRowDec :: String -> RowDecoder (Maybe a) -> RowDecoder a
+nonNullableRowDec haskellTypeName rdec =
+  let fromNullable mVal = case mVal of
+        Nothing -> fail $ "Cannot decode SQL null as the Haskell " ++ haskellTypeName ++ " type. Use a `" ++ haskellTypeName ++ "` if you want SQL nulls"
+        Just v -> pure v
+   in RowDecoder
+        { fullRowDecoder = \finfos -> rdec.fullRowDecoder finfos >>= fromNullable,
+          rowColumnsTypeCheck = rdec.rowColumnsTypeCheck,
+          numExpectedColumns = rdec.numExpectedColumns
+        }
+
 instance (FromPgField a) => FromPgField (Maybe a) where
   fieldDecoder = nullableField fieldDecoder
 
+-- TODO specialized row decoders! But can we?
+-- singleFieldRowDecoder = nullableRow inlinedSingleFieldRowDecoder
+
 allowOnlyArrayTypes :: FieldInfo -> Bool
 allowOnlyArrayTypes fieldInfo =
   -- TODO: We could check the elemTypeOid too, but maybe later

From 2e28392c6250173d3288375554dff140a969dacc Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Sun, 16 Aug 2026 10:10:10 -0300
Subject: [PATCH 12/22] Very experimental change with `Maybe a` instances

---
 Runfile                           |   2 +-
 hpgsql/src/Hpgsql/Encoding.hs     | 286 +++++++++++++-----------------
 hpgsql/src/Hpgsql/Internal.hs     |   2 +-
 hpgsql/src/Hpgsql/SimpleParser.hs |  16 +-
 hpgsql/src/Hpgsql/Types.hs        |  28 ++-
 5 files changed, 146 insertions(+), 188 deletions(-)

diff --git a/Runfile b/Runfile
index 406b180..1b85191 100644
--- a/Runfile
+++ b/Runfile
@@ -73,7 +73,7 @@ tests:
         if [ -n "$NIX" ]; then
             nix-build --no-out-link -A "testsPg${pg}" --argstr hspecArgs "$TARGS"
         else
-            cabal build hpgsql-tests hpgsql-simple-compat-tests
+            cabal build hpgsql-tests
             nix-shell -A "shellPg${pg}" ./default.nix --run "./scripts/run-tests-db-internal.sh $TARGS"
         fi
     done
diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index 7f32c19..5d25248 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -121,7 +121,7 @@ data FieldInfo = FieldInfo
 
 -- | A decoder for a single field/column.
 data FieldDecoder a = FieldDecoder
-  { fieldValueDecoder :: FieldInfo -> Maybe ByteString -> Either String a,
+  { fieldValueDecoder :: FieldInfo -> Maybe ByteString -> Either String (Maybe a),
     allowedPgTypes :: FieldInfo -> Bool
   }
   deriving stock (Functor)
@@ -157,7 +157,7 @@ instance Applicative RowDecoder where
 instance (TypeError (TypeLits.Text "RowDecoder does not have a Monad instance in Hpgsql because Hpgsql type-checks the result types of queries before having access to even the first data row. Use the Applicative class to write your instances or use the Monadic decoding variants.")) => Monad RowDecoder where
   (>>=) = error "inaccessible bind in Monad RowDecoder instance"
 
-{-# INLINE singleField #-} -- 1.2% wall time perf. gain with this
+{-# INLINE singleField #-}
 singleField :: FieldDecoder a -> RowDecoder a
 singleField (FieldDecoder {..}) =
   RowDecoder
@@ -172,7 +172,8 @@ singleField (FieldDecoder {..}) =
                       Just <$> Parser.take lenNextCol
                     else pure Nothing
                 case decode nextColBs of
-                  Right v -> pure v
+                  Right Nothing -> fail "Got SQL NULL but no nulls accepted" -- TODO: Please improve the failure message.. show field name and target Haskell type if we can
+                  Right (Just v) -> pure v
                   Left err -> fail err
         _ -> error "singleField expected a single column OID but got 0 or >1",
       rowColumnsTypeCheck = \case
@@ -244,9 +245,9 @@ compositeTypeDecoder :: forall a. RowDecoder a -> FieldDecoder a
 compositeTypeDecoder (RowDecoder {..}) =
   FieldDecoder
     { fieldValueDecoder = \compositeTypeOid -> \case
-        Nothing -> Left "Got NULL in composite type but it was not allowed"
-        Just bs -> case Parser.parseOnly (parserForRecord compositeTypeOid.encodingContext <* Parser.endOfInput) bs of
-          Parser.ParseOk v -> Right v
+        Nothing -> Right Nothing -- Left "Got NULL in composite type but it was not allowed"
+        Just bs -> case Parser.parseOnly (parserForRecord compositeTypeOid.encodingContext <* Parser.endOfInput "compositeTypeDecoder") bs of
+          Parser.ParseOk v -> Right (Just v)
           Parser.ParseFail err -> Left err,
       allowedPgTypes = const True -- There's no way to enforce a custom type's OID. We only check if it's structurally the same in the parser (same subtypes in same order)
     }
@@ -265,7 +266,7 @@ compositeTypeDecoder (RowDecoder {..}) =
         pure (oid, sizeBs <> bs)
       let typecheckedCols = rowColumnsTypeCheck (map (mkColInfo . fst) cols)
       unless (all snd typecheckedCols) $ fail $ "Parser for composite found type OIDs " ++ show (map fst cols) ++ " but expected different"
-      case Parser.parseOnly (fullRowDecoder (map (mkColInfo . fst) cols) <* Parser.endOfInput) (mconcat $ map snd cols) of
+      case Parser.parseOnly (fullRowDecoder (map (mkColInfo . fst) cols) <* Parser.endOfInput "parserForRecord") (mconcat $ map snd cols) of
         Parser.ParseOk v -> pure v
         Parser.ParseFail err -> error $ "Error decoding composite type: " ++ show err
 
@@ -782,7 +783,7 @@ binaryFloat4Decoder = castWord32ToFloat . either error id . BinSer.decodeWord32B
 binaryFloat8Decoder :: ByteString -> Double
 binaryFloat8Decoder = castWord64ToDouble . either error id . BinSer.decodeWord64BE 0
 
-parsePgType :: [Oid] -> (Maybe ByteString -> Either String a) -> FieldDecoder a
+parsePgType :: [Oid] -> (Maybe ByteString -> Either String (Maybe a)) -> FieldDecoder a
 parsePgType !requiredTypeOids !fieldValueDecoder =
   FieldDecoder
     { fieldValueDecoder = \_oid -> fieldValueDecoder,
@@ -793,9 +794,9 @@ instance FromPgField () where
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \_oid -> \case
-          Just "" -> Right ()
+          Just "" -> Right (Just ())
           Just bs -> Left $ "Invalid value '" ++ show bs ++ "' for postgres void type"
-          Nothing -> Left "Cannot decode SQL null as the Haskell () type. Use a `Maybe ()`",
+          Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell () type. Use a `Maybe ()`",
         allowedPgTypes = (== voidOid) . fieldTypeOid
       }
 
@@ -819,8 +820,8 @@ instance FromPgField Int where
       { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
           let !decode = binaryIntDecoder oid
            in \case
-                Just bs -> decode bs
-                Nothing -> Left "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`",
+                Just bs -> Just <$> decode bs
+                Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`",
         allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
@@ -828,32 +829,14 @@ instance FromPgField Int where
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Int" intRowDecoder
 
-instance {-# OVERLAPPING #-} FromPgField (Maybe Int) where
-  -- This overlapping instance isn't pretty, but it reduces memory
-  -- usage and improves performance a bit
-  fieldDecoder = error "TODO Maybe Int"
-
-  -- FieldDecoder
-  --   { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
-  --       let !decode = binaryIntDecoder oid
-  --        in \case
-  --             Just bs -> Just <$> decode bs
-  --             Nothing -> Right Nothing,
-  --     allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
-  --   }
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = intRowDecoder
-
 instance FromPgField Int16 where
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder =
           let !decode = binaryIntDecoder int2Oid
            in const $ \case
-                Just bs -> decode bs
-                Nothing -> Left "Cannot decode SQL null as the Haskell Int16 type. Use a `Maybe Int16`",
+                Just bs -> Just <$> decode bs
+                Nothing -> Right Nothing, -- Left "Cannot decode SQL null as the Haskell Int16 type. Use a `Maybe Int16`",
         allowedPgTypes = (== int2Oid) . fieldTypeOid
       }
 
@@ -874,8 +857,8 @@ instance FromPgField Int32 where
       { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
           let !decode = binaryIntDecoder oid
            in \case
-                Just bs -> decode bs
-                Nothing -> Left "Cannot decode SQL null as the Haskell Int32 type. Use a `Maybe Int32`",
+                Just bs -> Just <$> decode bs
+                Nothing -> Right Nothing, -- Left "Cannot decode SQL null as the Haskell Int32 type. Use a `Maybe Int32`",
         allowedPgTypes = (`elem` [int2Oid, int4Oid]) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
@@ -901,8 +884,8 @@ instance FromPgField Int64 where
       { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
           let !decode = binaryIntDecoder oid
            in \case
-                Just bs -> decode bs
-                Nothing -> Left "Cannot decode SQL null as the Haskell Int64 type. Use a `Maybe Int64`",
+                Just bs -> Just <$> decode bs
+                Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell Int64 type. Use a `Maybe Int64`",
         allowedPgTypes = (`elem` [int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
@@ -917,13 +900,13 @@ instance FromPgField Integer where
           let !decodeInt = binaryIntDecoder @Int64 oid
            in \case
                 Just bs
-                  | oid /= numericOid -> fromIntegral <$> decodeInt bs
-                  | otherwise -> case Parser.parseOnly (scientificDecoder True <* Parser.endOfInput) bs of
+                  | oid /= numericOid -> Just . fromIntegral <$> decodeInt bs
+                  | otherwise -> case Parser.parseOnly (scientificDecoder True <* Parser.endOfInput "Integer") bs of
                       Parser.ParseOk sci -> case floatingOrInteger @Double @Integer sci of
-                        Right i -> Right i
+                        Right i -> Right (Just i)
                         Left _ -> Left "Internal error in Hpgsql. Scientific to Integer conversion failed"
                       Parser.ParseFail err -> Left err
-                Nothing -> Left "Cannot decode SQL null as the Haskell Integer type. Use a `Maybe Integer`",
+                Nothing -> Right Nothing, -- Left "Cannot decode SQL null as the Haskell Integer type. Use a `Maybe Integer`",
         allowedPgTypes = (`elem` [int8Oid, numericOid, int4Oid, int2Oid]) . fieldTypeOid
       }
 
@@ -932,8 +915,8 @@ instance FromPgField Oid where
     FieldDecoder
       { fieldValueDecoder = \_ -> \case
           -- Oids are just int4
-          Just bs -> Oid <$> binaryIntDecoder int4Oid bs
-          Nothing -> Left "Cannot decode SQL null as the Haskell Oid type. Use a `Maybe Oid`",
+          Just bs -> Just . Oid <$> binaryIntDecoder int4Oid bs
+          Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell Oid type. Use a `Maybe Oid`",
         allowedPgTypes = (== oidOid) . fieldTypeOid
       }
 
@@ -944,22 +927,13 @@ floatRowDecoder =
 
 instance FromPgField Float where
   fieldDecoder = parsePgType [float4Oid] $ \case
-    Just bs -> Right $ binaryFloat4Decoder bs
-    Nothing -> Left "Cannot decode SQL null as the Haskell Float type. Use a `Maybe Float`"
+    Just bs -> Right $ Just $ binaryFloat4Decoder bs
+    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell Float type. Use a `Maybe Float`"
   {-# NOINLINE singleFieldRowDecoder #-}
   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Float" floatRowDecoder
 
-instance {-# OVERLAPPING #-} FromPgField (Maybe Float) where
-  -- This overlapping instance isn't pretty, but it reduces memory
-  -- usage and improves performance a bit
-  fieldDecoder = error "TODO Maybe Float"
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = floatRowDecoder
-
 {-# INLINE doubleRowDecoder #-}
 doubleRowDecoder :: RowDecoder (Maybe Double)
 doubleRowDecoder =
@@ -979,8 +953,8 @@ instance FromPgField Double where
                 | oid == float8Oid = binaryFloat8Decoder
                 | otherwise = float2Double . binaryFloat4Decoder
            in \case
-                Just bs -> Right $ decoder bs
-                Nothing -> Left "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`",
+                Just bs -> Right $ Just $ decoder bs
+                Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`",
         allowedPgTypes = (`elem` [float8Oid, float4Oid]) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
@@ -988,15 +962,6 @@ instance FromPgField Double where
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Double" doubleRowDecoder
 
-instance {-# OVERLAPPING #-} FromPgField (Maybe Double) where
-  -- This overlapping instance isn't pretty, but it reduces memory
-  -- usage and improves performance a bit
-  fieldDecoder = error "TODO Maybe Double"
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = doubleRowDecoder
-
 -- | Allows you to specify a type (and other checks, possibly) for a `FieldDecoder`.
 -- This can be useful to ensure you're not accidentally decoding a different type.
 --
@@ -1055,16 +1020,16 @@ instance FromPgField Scientific where
             then
               let intdec = binaryIntDecoder @Int64 fieldTypeOid
                in \case
-                    Just bs -> flip scientific 0 . fromIntegral <$> intdec bs
-                    Nothing -> Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`"
+                    Just bs -> Just . flip scientific 0 . fromIntegral <$> intdec bs
+                    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`"
             else \case
               Just bs ->
                 -- TODO: There is loss converting from Float/Double to Scientific, but it might be quite small, so should we accept
                 -- float4Oid and float8Oid here?
-                case Parser.parseOnly (scientificDecoder False <* Parser.endOfInput) bs of
-                  Parser.ParseOk sci -> Right sci
+                case Parser.parseOnly (scientificDecoder False <* Parser.endOfInput "Scientific") bs of
+                  Parser.ParseOk sci -> Right (Just sci)
                   Parser.ParseFail err -> Left err
-              Nothing -> Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`",
+              Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`",
         allowedPgTypes = (`elem` [numericOid, int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
@@ -1096,22 +1061,13 @@ boolRowDecoder =
 
 instance FromPgField Bool where
   fieldDecoder = parsePgType [boolOid] $ \case
-    Just bs -> Right $ bs == binaryTrue
-    Nothing -> Left "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
+    Just bs -> Right $ Just $ bs == binaryTrue
+    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
   {-# NOINLINE singleFieldRowDecoder #-}
   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Bool" boolRowDecoder
 
-instance {-# OVERLAPPING #-} FromPgField (Maybe Bool) where
-  -- This overlapping instance isn't pretty, but it reduces memory
-  -- usage and improves performance a bit
-  fieldDecoder = error "TODO Maybe Bool"
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = boolRowDecoder
-
 instance FromPgField Char where
   fieldDecoder =
     let textParser = fieldValueDecoder (fieldDecoder @Text)
@@ -1123,24 +1079,21 @@ instance FromPgField Char where
                       if oid == charOid
                         -- TODO: Postgres has values of type "char" in the pg_type.typcategory table.
                         -- We should test this instance works with those, and we haven't yet.
-                        then Right $ BSC.head bs
+                        then Right $ Just $ BSC.head bs
                         else case decodeText mbs of
                           Left err -> Left err
-                          Right t -> if Text.length t > 1 then Left "Cannot parse text with more than one character into a Haskell Char type." else Right (Text.head t)
-                    Nothing -> Left "Cannot decode SQL null as the Haskell Char type. Use a `Maybe Char`",
-            -- TODO: All the varchar types?
+                          Right (Just t) -> if Text.length t > 1 then Left "Cannot parse text with more than one character into a Haskell Char type." else Right (Just $ Text.head t)
+                          Right Nothing -> Right Nothing
+                    Nothing -> Right Nothing, -- Left "Cannot decode SQL null as the Haskell Char type. Use a `Maybe Char`",
+                    -- TODO: All the varchar types?
             allowedPgTypes = (`elem` [charOid, textOid]) . fieldTypeOid
           }
 
 instance FromPgField ByteString where
-  fieldDecoder = parsePgType [byteaOid] $ \case
-    Just bs -> Right bs
-    Nothing -> Left "Cannot decode SQL null as the Haskell ByteString type. Use a `Maybe ByteString`"
+  fieldDecoder = parsePgType [byteaOid] Right
 
 instance FromPgField LBS.ByteString where
-  fieldDecoder = parsePgType [byteaOid] $ \case
-    Just bs -> Right $ LBS.fromStrict bs
-    Nothing -> Left "Cannot decode SQL null as the Haskell ByteString type. Use a `Maybe ByteString`"
+  fieldDecoder = parsePgType [byteaOid] $ Right . fmap LBS.fromStrict
 
 {-# INLINE textDecoder #-}
 textDecoder :: RowDecoder (Maybe Text)
@@ -1154,35 +1107,26 @@ textDecoder =
 
 instance FromPgField Text where
   fieldDecoder = parsePgType [textOid, varcharOid, nameOid] $ \case
-    Just bs -> Right $ decodeUtf8 bs
+    Just bs -> Right $ Just $ decodeUtf8 bs
     -- TODO: Use some faster unsafeDecodeUtf8 function?
-    Nothing -> Left "Cannot decode SQL null as the Haskell Text type. Use a `Maybe Text`"
+    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell Text type. Use a `Maybe Text`"
   {-# NOINLINE singleFieldRowDecoder #-}
   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Text" textDecoder
 
-instance {-# OVERLAPPING #-} FromPgField (Maybe Text) where
-  -- This overlapping instance isn't pretty, but it reduces memory
-  -- usage and improves performance a bit
-  fieldDecoder = error "TODO Maybe Text"
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = textDecoder
-
 instance FromPgField LT.Text where
   fieldDecoder = parsePgType [textOid, varcharOid, nameOid] $ \case
-    Just bs -> Right $ LT.fromStrict $ decodeUtf8 bs
+    Just bs -> Right $ Just $ LT.fromStrict $ decodeUtf8 bs
     -- TODO: Use some faster unsafeDecodeUtf8 function?
-    Nothing -> Left "Cannot decode SQL null as the Haskell Text type. Use a `Maybe Text`"
+    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell Text type. Use a `Maybe Text`"
 
 instance FromPgField String where
   fieldDecoder = parsePgType [textOid, varcharOid, nameOid] $ \case
     -- connection option).
-    Just bs -> Right $ Text.unpack $ decodeUtf8 bs
+    Just bs -> Right $ Just $ Text.unpack $ decodeUtf8 bs
     -- TODO: Use some faster unsafeDecodeUtf8 function?
-    Nothing -> Left "Cannot decode SQL null as the Haskell String type. Use a `Maybe String`"
+    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell String type. Use a `Maybe String`"
 
 -- | This instance does not work if you have fillTypeInfoCache disabled (that would be a non-default
 -- connection option).
@@ -1219,22 +1163,13 @@ instance FromPgField UTCTime where
       totalusecs <- BinSer.decodeInt64BE 0 bs
       let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
           parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-      Right $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-    Nothing -> Left "Cannot decode SQL null as the Haskell UTCTime type. Use a `Maybe UTCTime`"
+      Right $ Just $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
+    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell UTCTime type. Use a `Maybe UTCTime`"
   {-# NOINLINE singleFieldRowDecoder #-}
   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# NOINLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "UTCTime" utcTimeRowDecoder
 
-instance {-# OVERLAPPING #-} FromPgField (Maybe UTCTime) where
-  -- This overlapping instance isn't pretty, but it reduces memory
-  -- usage and improves performance a bit
-  fieldDecoder = error "TODO Maybe UTCTime"
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = utcTimeRowDecoder
-
 instance FromPgField (Unbounded UTCTime) where
   fieldDecoder = parsePgType [timestamptzOid] $ \case
     Just bs -> do
@@ -1242,15 +1177,15 @@ instance FromPgField (Unbounded UTCTime) where
       totalusecs <- BinSer.decodeInt64BE 0 bs
       Right $
         if totalusecs == minBound
-          then NegInfinity
+          then Just NegInfinity
           else
             if totalusecs == maxBound
-              then PosInfinity
+              then Just PosInfinity
               else
                 let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
                     parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-                 in Finite $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-    Nothing -> Left "Cannot decode SQL null as the Haskell (Unbounded UTCTime) type. Use a `Maybe (Unbounded UTCTime)`"
+                 in Just $ Finite $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
+    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell (Unbounded UTCTime) type. Use a `Maybe (Unbounded UTCTime)`"
 
 instance FromPgField ZonedTime where
   fieldDecoder = parsePgType [timestamptzOid] $ \case
@@ -1259,8 +1194,8 @@ instance FromPgField ZonedTime where
       totalusecs <- BinSer.decodeInt64BE 0 bs
       let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
           parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-      Right $ utcToZonedTime utc $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-    Nothing -> Left "Cannot decode SQL null as the Haskell ZonedTime type. Use a `Maybe ZonedTime`"
+      Right $ Just $ utcToZonedTime utc $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
+    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell ZonedTime type. Use a `Maybe ZonedTime`"
 
 instance FromPgField (Unbounded ZonedTime) where
   fieldDecoder = parsePgType [timestamptzOid] $ \case
@@ -1269,15 +1204,15 @@ instance FromPgField (Unbounded ZonedTime) where
       totalusecs <- BinSer.decodeInt64BE 0 bs
       Right $
         if totalusecs == minBound
-          then NegInfinity
+          then Just NegInfinity
           else
             if totalusecs == maxBound
-              then PosInfinity
+              then Just PosInfinity
               else
                 let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
                     parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-                 in Finite $ utcToZonedTime utc $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-    Nothing -> Left "Cannot decode SQL null as the Haskell ZonedTime type. Use a `Maybe ZonedTime`"
+                 in Just $ Finite $ utcToZonedTime utc $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
+    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell ZonedTime type. Use a `Maybe ZonedTime`"
 
 instance FromPgField LocalTime where
   fieldDecoder = parsePgType [timestampOid] $ \case
@@ -1285,15 +1220,15 @@ instance FromPgField LocalTime where
       totalusecs <- BinSer.decodeInt64BE 0 bs
       let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
           parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-      Right $ LocalTime parsedDate (timeToTimeOfDay $ picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-    Nothing -> Left "Cannot decode SQL null as the Haskell LocalTime type. Use a `Maybe LocalTime`"
+      Right $ Just $ LocalTime parsedDate (timeToTimeOfDay $ picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
+    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell LocalTime type. Use a `Maybe LocalTime`"
 
 instance FromPgField TimeOfDay where
   fieldDecoder = parsePgType [timeOid] $ \case
     Just bs -> do
       usecs <- BinSer.decodeInt64BE 0 bs
-      Right $ timeToTimeOfDay $ picosecondsToDiffTime $ fromIntegral usecs * 1_000_000
-    Nothing -> Left "Cannot decode SQL null as the Haskell TimeOfDay type. Use a `Maybe TimeOfDay`"
+      Right $ Just $ timeToTimeOfDay $ picosecondsToDiffTime $ fromIntegral usecs * 1_000_000
+    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell TimeOfDay type. Use a `Maybe TimeOfDay`"
 
 {-# INLINE dayRowDecoder #-}
 dayRowDecoder :: RowDecoder (Maybe Day)
@@ -1308,22 +1243,13 @@ instance FromPgField Day where
       -- https://github.com/postgres/postgres/blob/799959dc7cf0e2462601bea8d07b6edec3fa0c4f/src/backend/utils/adt/datetime.c#L321
       -- But I found a simpler way to do this. Let's see if it works in our property based tests
       jd <- BinSer.decodeInt32BE 0 bs
-      Right $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
-    Nothing -> Left "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
+      Right $ Just $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
+    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
   {-# NOINLINE singleFieldRowDecoder #-}
   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Day" dayRowDecoder
 
-instance {-# OVERLAPPING #-} FromPgField (Maybe Day) where
-  -- This overlapping instance isn't pretty, but it reduces memory
-  -- usage and improves performance a bit
-  fieldDecoder = error "TODO Maybe Day"
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = dayRowDecoder
-
 instance FromPgField (Unbounded Day) where
   fieldDecoder = parsePgType [dateOid] $ \case
     Just bs -> do
@@ -1333,13 +1259,13 @@ instance FromPgField (Unbounded Day) where
       jd <- BinSer.decodeInt32BE 0 bs
       Right $
         if jd == minBound
-          then NegInfinity
+          then Just NegInfinity
           else
             if jd == maxBound
-              then PosInfinity
+              then Just PosInfinity
               else
-                Finite $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
-    Nothing -> Left "Cannot decode SQL null as the Haskell (Unbounded Day) type. Use a `Maybe (Unbounded Day)`"
+                Just $ Finite $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
+    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell (Unbounded Day) type. Use a `Maybe (Unbounded Day)`"
 
 instance FromPgField CalendarDiffTime where
   fieldDecoder = parsePgType [intervalOid] $ \case
@@ -1347,15 +1273,15 @@ instance FromPgField CalendarDiffTime where
       nMicrosecs <- BinSer.decodeInt64BE 0 bs
       nDays <- BinSer.decodeInt32BE 8 bs
       nMonths <- BinSer.decodeInt32BE 12 bs
-      Right $ CalendarDiffTime {ctMonths = fromIntegral nMonths, ctTime = secondsToNominalDiffTime (fromIntegral nDays * 86400) + realToFrac (picosecondsToDiffTime (fromIntegral nMicrosecs * 1_000_000))}
-    Nothing -> Left "Cannot decode SQL null as the Haskell CalendarDiffTime type. Use a `Maybe CalendarDiffTime`"
+      Right $ Just $ CalendarDiffTime {ctMonths = fromIntegral nMonths, ctTime = secondsToNominalDiffTime (fromIntegral nDays * 86400) + realToFrac (picosecondsToDiffTime (fromIntegral nMicrosecs * 1_000_000))}
+    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell CalendarDiffTime type. Use a `Maybe CalendarDiffTime`"
 
 instance FromPgField UUID where
   fieldDecoder = parsePgType [uuidOid] $ \case
     Just bs -> case UUID.fromByteString (LBS.fromStrict bs) of
-      Just uuid -> Right uuid
+      Just uuid -> Right (Just uuid)
       Nothing -> Left "Bug in Hpgsql: UUID field could not be decoded"
-    Nothing -> Left "Cannot decode SQL null as the Haskell UUID type. Use a `Maybe UUID`"
+    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell UUID type. Use a `Maybe UUID`"
 
 instance FromPgField Aeson.Value where
   fieldDecoder =
@@ -1363,12 +1289,14 @@ instance FromPgField Aeson.Value where
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
             let -- jsonb has a byte prepended to the contents and json does not
+                -- TODO: Does `BS.drop 1` get inlined into `Aeson.decodeStrict` further down?
+                -- Might be an interesting case study
                 !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
              in \case
-                  Just bs -> case Aeson.decodeStrict $ fixJsonb bs of
-                    Just d -> Right d
+                  Just bs -> case Aeson.decodeStrict @Aeson.Value $ fixJsonb bs of
+                    Just d -> Right (Just d)
                     Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid."
-                  Nothing -> Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
+                  Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
 
@@ -1380,6 +1308,13 @@ nullableField FieldDecoder {..} =
     { fieldValueDecoder = \oid ->
         let !origFieldValueParser = fieldValueDecoder oid
          in \case
+              -- TODO: we have multiple layers of Maybe/NULL here.
+              -- What should we do? If the supplied decoder returns
+              -- Nothing, do we return `Just Nothing`? Or do we follow
+              -- SQL's viral NULL semantics?
+              -- Think about a custom user type that decodes NULL into a Just,
+              -- and test that with all decoding combinations we provide (
+              -- singleField, nullableField, specialized inlined/not-inlined row decoders, etc.)
               Nothing -> Right Nothing
               justBs -> Just <$> origFieldValueParser justBs,
       allowedPgTypes
@@ -1397,11 +1332,30 @@ nonNullableRowDec haskellTypeName rdec =
           numExpectedColumns = rdec.numExpectedColumns
         }
 
+{-# INLINE nullableRowDec #-}
+nullableRowDec :: RowDecoder a -> RowDecoder (Maybe a)
+nullableRowDec rdec =
+  RowDecoder
+    { fullRowDecoder = \finfos -> do
+        -- Peek the field length, but only consume it
+        -- if we get a NULL. Urgh.
+        fieldLen <- Parser.peekInt32BE
+        if fieldLen == (-1)
+          then do
+            Parser.skip 4
+            pure Nothing
+          else
+            Just <$> rdec.fullRowDecoder finfos,
+      rowColumnsTypeCheck = rdec.rowColumnsTypeCheck,
+      numExpectedColumns = rdec.numExpectedColumns
+    }
+
 instance (FromPgField a) => FromPgField (Maybe a) where
   fieldDecoder = nullableField fieldDecoder
-
--- TODO specialized row decoders! But can we?
--- singleFieldRowDecoder = nullableRow inlinedSingleFieldRowDecoder
+  {-# NOINLINE singleFieldRowDecoder #-}
+  singleFieldRowDecoder = nullableRowDec inlinedSingleFieldRowDecoder
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder = nullableRowDec inlinedSingleFieldRowDecoder
 
 allowOnlyArrayTypes :: FieldInfo -> Bool
 allowOnlyArrayTypes fieldInfo =
@@ -1419,11 +1373,11 @@ instance {-# OVERLAPPING #-} forall a. (FromPgField a) => FromPgField (Vector (V
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \colInfo ->
-          let !arrayFieldDecoder = arrayParser colInfo.encodingContext <* Parser.endOfInput
+          let !arrayFieldDecoder = arrayParser colInfo.encodingContext <* Parser.endOfInput "Vector (Vector a)"
            in \case
-                Nothing -> Left "Cannot decode SQL null as the Haskell (Vector (Vector a)) type. Use a `Maybe (Vector (Vector a))`"
+                Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell (Vector (Vector a)) type. Use a `Maybe (Vector (Vector a))`"
                 Just bs -> case Parser.parseOnly arrayFieldDecoder bs of
-                  Parser.ParseOk v -> Right v
+                  Parser.ParseOk v -> Right (Just v)
                   Parser.ParseFail err -> Left err,
         allowedPgTypes = allowOnlyArrayTypes
       }
@@ -1453,7 +1407,8 @@ instance {-# OVERLAPPING #-} forall a. (FromPgField a) => FromPgField (Vector (V
               elementBs <- if size == (-1) then pure Nothing else Just <$> Parser.take size
               case elementParser.fieldValueDecoder elementColInfo elementBs of
                 Left err -> fail $ "Error parsing array element: " ++ show err
-                Right el -> pure el
+                Right (Just el) -> pure el
+                Right Nothing -> fail "Found an array element that is NULL" -- TODO: I think this is a bug of ours, because what if a ~ Maybe b?
 
 {-# INLINE genericFromPgRow #-}
 
@@ -1585,9 +1540,7 @@ untypedFieldEncoder enc = FieldEncoder {toTypeOid = \_ -> Nothing, toPgField = e
 rawBytesFieldDecoder :: FieldDecoder ByteString
 rawBytesFieldDecoder =
   FieldDecoder
-    { fieldValueDecoder = \_oid -> \case
-        Nothing -> Left "Cannot decode SQL null as the `rawBytesFieldDecoder`."
-        Just bs -> Right bs,
+    { fieldValueDecoder = const Right,
       allowedPgTypes = const True
     }
 
@@ -1615,11 +1568,11 @@ arrayField !replicateFunction !elementParser =
   -- From https://github.com/postgres/postgres/blob/5941946d0934b9eccb0d5bfebd40b155249a0130/src/backend/utils/adt/arrayfuncs.c#L1548
   FieldDecoder
     { fieldValueDecoder = \colInfo ->
-        let !arrayFieldDecoder = arrayParser colInfo.encodingContext <* Parser.endOfInput
+        let !arrayFieldDecoder = arrayParser colInfo.encodingContext <* Parser.endOfInput "arrayField"
          in \case
-              Nothing -> Left "Cannot decode SQL null as the Haskell Vector type. Use a `Maybe (Vector a)`"
+              Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell Vector type. Use a `Maybe (Vector a)`"
               Just bs -> case Parser.parseOnly arrayFieldDecoder bs of
-                Parser.ParseOk v -> Right v
+                Parser.ParseOk v -> Right (Just v)
                 Parser.ParseFail err -> Left err,
       allowedPgTypes = allowOnlyArrayTypes
     }
@@ -1642,4 +1595,5 @@ arrayField !replicateFunction !elementParser =
             elementBs <- if size == (-1) then pure Nothing else Just <$> Parser.take size
             case elementParser.fieldValueDecoder elementColInfo elementBs of
               Left err -> fail $ "Error parsing array element: " ++ show err
-              Right el -> pure el
+              Right (Just el) -> pure el
+              Right Nothing -> fail "Found an array element that is NULL" -- TODO: I think this is a bug of ours, because what if a ~ Maybe b?
diff --git a/hpgsql/src/Hpgsql/Internal.hs b/hpgsql/src/Hpgsql/Internal.hs
index 8c427fe..4cfa3c0 100644
--- a/hpgsql/src/Hpgsql/Internal.hs
+++ b/hpgsql/src/Hpgsql/Internal.hs
@@ -1418,7 +1418,7 @@ consumeStreamingResults rp conn qryId = S.effect $ do
           S.concat $
             S.mapM
               ( \(DataRows rowColumnData) ->
-                  case Parser.parseOnly (Parser.parseMany rowparser <* Parser.endOfInput) rowColumnData of
+                  case Parser.parseOnly (Parser.parseMany rowparser <* Parser.endOfInput "DataRows") rowColumnData of
                     Parser.ParseOk rows -> pure rows
                     Parser.ParseFail err -> throwIrrecoverableErrorWithStatement qText $ "Failed parsing a row: " <> Text.pack (show err)
               )
diff --git a/hpgsql/src/Hpgsql/SimpleParser.hs b/hpgsql/src/Hpgsql/SimpleParser.hs
index 145c54d..28fec4d 100644
--- a/hpgsql/src/Hpgsql/SimpleParser.hs
+++ b/hpgsql/src/Hpgsql/SimpleParser.hs
@@ -31,6 +31,7 @@ module Hpgsql.SimpleParser
     takeFloatBE,
     takeDoubleBE,
     takeFloatBEWithFieldLength,
+    peekInt32BE,
   )
 where
 
@@ -39,7 +40,7 @@ import Data.ByteString (ByteString)
 import qualified Data.ByteString as BS
 import Data.Int (Int16, Int32, Int64)
 import Foreign.Storable (Storable)
-import GHC.Float (castWord32ToFloat, castWord64ToDouble, word2Float)
+import GHC.Float (castWord32ToFloat, castWord64ToDouble)
 import Hpgsql.Encoding.BinarySerializer (ByteStringIdx (..))
 import qualified Hpgsql.Encoding.BinarySerializer as BinSer
 import Prelude hiding (take)
@@ -152,6 +153,13 @@ takeInt32BE = Parser $ \idx bs kf ks ->
     Right v -> ks v (idx + 4) bs
     Left err -> kf err
 
+{-# INLINE peekInt32BE #-}
+peekInt32BE :: Parser Int32
+peekInt32BE = Parser $ \idx bs kf ks ->
+  case BinSer.decodeInt32BE idx bs of
+    Right v -> ks v idx bs
+    Left err -> kf err
+
 {-# INLINE takeInt32BEWithFieldLength #-}
 
 -- | Parses both a field length and the field itself, for
@@ -241,9 +249,9 @@ parseManyRows = Parser $ \idx' bs' _kf ks -> let restIdx = go idx' bs' in ks res
 {-# INLINE parseManyRows #-}
 
 -- | Succeeds only when the input has been fully consumed.
-endOfInput :: Parser ()
-endOfInput = Parser $ \idx bs kf ks ->
-  if BS.length bs <= idx.idx then ks () idx bs else kf "endOfInput: input remaining"
+endOfInput :: String -> Parser ()
+endOfInput debugFail = Parser $ \idx bs kf ks ->
+  if BS.length bs <= idx.idx then ks () idx bs else kf $ "endOfInput: input remaining (" ++ debugFail ++ ")"
 {-# INLINE endOfInput #-}
 
 -- | Run a parser and additionally return the slice of input it consumed.
diff --git a/hpgsql/src/Hpgsql/Types.hs b/hpgsql/src/Hpgsql/Types.hs
index 21fb011..ad19e16 100644
--- a/hpgsql/src/Hpgsql/Types.hs
+++ b/hpgsql/src/Hpgsql/Types.hs
@@ -87,13 +87,11 @@ instance FromPgField PgJson where
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let
-              -- jsonb has a byte prepended to the contents and json does not
-              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in
-              \case
-                Just bs -> Right $ PgJson $ fixJsonb bs
-                Nothing -> Left "Cannot decode SQL null as the Haskell PgJson type. Use a `Maybe PgJson` if you want SQL nulls",
+            let -- jsonb has a byte prepended to the contents and json does not
+                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in \case
+                  Just bs -> Right $ Just $ PgJson $ fixJsonb bs
+                  Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell PgJson type. Use a `Maybe PgJson` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
 
@@ -109,15 +107,13 @@ instance (FromJSON a) => FromPgField (Aeson a) where
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let
-              -- jsonb has a byte prepended to the contents and json does not
-              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in
-              \case
-                Just bs -> case Aeson.decodeStrict $ fixJsonb bs of
-                  Just v -> Right $ Aeson v
-                  Nothing -> Left "Failed to decode postgres JSON value into your `Aeson a` type. Are you sure it's proper JSON?"
-                Nothing -> Left "Cannot decode SQL null as a Haskell (Aeson a) type. Use a `Maybe (Aeson a)` if you want SQL nulls",
+            let -- jsonb has a byte prepended to the contents and json does not
+                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in \case
+                  Just bs -> case Aeson.decodeStrict $ fixJsonb bs of
+                    Just v -> Right $ Just $ Aeson v
+                    Nothing -> Left "Failed to decode postgres JSON value into your `Aeson a` type. Are you sure it's proper JSON?"
+                  Nothing -> Right Nothing, -- Left "Cannot decode SQL null as a Haskell (Aeson a) type. Use a `Maybe (Aeson a)` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
 

From fbfa92b708fac7c1ebe8077f06f6210396ea33a7 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Mon, 17 Aug 2026 15:34:12 -0300
Subject: [PATCH 13/22] A separate field in FieldDecoder for what to decode
 NULL to

With this, I am able to write specialised `FromPgField (Maybe a)`
instances that can inline more aggressively.

The gotcha is
---
 Runfile                           |   6 +-
 hpgsql-tests/RowDecoderGhcCore.hs |   2 +-
 hpgsql/src/Hpgsql/Encoding.hs     | 455 +++++++++++++++++-------------
 hpgsql/src/Hpgsql/Internal.hs     |   2 +-
 hpgsql/src/Hpgsql/SimpleParser.hs |   6 +-
 hpgsql/src/Hpgsql/Types.hs        |  14 +-
 6 files changed, 267 insertions(+), 218 deletions(-)

diff --git a/Runfile b/Runfile
index 1b85191..0a791b9 100644
--- a/Runfile
+++ b/Runfile
@@ -73,13 +73,13 @@ tests:
         if [ -n "$NIX" ]; then
             nix-build --no-out-link -A "testsPg${pg}" --argstr hspecArgs "$TARGS"
         else
-            cabal build hpgsql-tests
+            cabal build hpgsql-tests # hpgsql-simple-compat-tests
             nix-shell -A "shellPg${pg}" ./default.nix --run "./scripts/run-tests-db-internal.sh $TARGS"
         fi
     done
 
-    echo "--- Running hlint"
-    hlint .
+    # echo "--- Running hlint"
+    # hlint .
 
 ##
 # Runs tests 100 times, reporting how many passed and how many failed.
diff --git a/hpgsql-tests/RowDecoderGhcCore.hs b/hpgsql-tests/RowDecoderGhcCore.hs
index 244ec4d..512efc8 100644
--- a/hpgsql-tests/RowDecoderGhcCore.hs
+++ b/hpgsql-tests/RowDecoderGhcCore.hs
@@ -27,7 +27,7 @@ import Hpgsql.Encoding (FromPgField (..), FromPgRow (..), genericFromPgRow, sing
 data BestCaseScenarioRecord = BestCaseScenarioRecord
   { bcsId :: !Int,
     bcsDate :: !Day,
-    bcsText :: !Int
+    bcsText :: !(Maybe Int)
   }
 
 instance FromPgRow BestCaseScenarioRecord where
diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index 5d25248..fc9513e 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -121,7 +121,8 @@ data FieldInfo = FieldInfo
 
 -- | A decoder for a single field/column.
 data FieldDecoder a = FieldDecoder
-  { fieldValueDecoder :: FieldInfo -> Maybe ByteString -> Either String (Maybe a),
+  { fieldValueDecoder :: FieldInfo -> ByteString -> Either String a, -- TODO: Since this now takes a ByteString (not a Maybe), it could actually be typed `FieldInfo -> Parser a`
+    decodesSqlNullTo :: Either String a,
     allowedPgTypes :: FieldInfo -> Bool
   }
   deriving stock (Functor)
@@ -137,6 +138,7 @@ instance Semigroup (FieldDecoder a) where
                 let cand1 = if dec1.allowedPgTypes cInfo then f1 mbs else Left "Not first parser"
                     cand2 = if dec2.allowedPgTypes cInfo then f2 mbs else Left "Not second parser"
                  in cand1 <> cand2,
+        decodesSqlNullTo = dec1.decodesSqlNullTo <> dec2.decodesSqlNullTo,
         allowedPgTypes = \cInfo -> dec1.allowedPgTypes cInfo || dec2.allowedPgTypes cInfo
       }
 
@@ -157,27 +159,27 @@ instance Applicative RowDecoder where
 instance (TypeError (TypeLits.Text "RowDecoder does not have a Monad instance in Hpgsql because Hpgsql type-checks the result types of queries before having access to even the first data row. Use the Applicative class to write your instances or use the Monadic decoding variants.")) => Monad RowDecoder where
   (>>=) = error "inaccessible bind in Monad RowDecoder instance"
 
-{-# INLINE singleField #-}
+{-# INLINE singleField #-} -- 1.2% wall time perf. gain with this
 singleField :: FieldDecoder a -> RowDecoder a
-singleField (FieldDecoder {..}) =
+singleField fdec =
   RowDecoder
     { fullRowDecoder = \case
         [singleColInfo] ->
-          let decode = fieldValueDecoder singleColInfo
+          let decode = fdec.fieldValueDecoder singleColInfo
            in do
                 lenNextCol <- fromIntegral <$> Parser.takeInt32BE
-                nextColBs <-
-                  if lenNextCol >= 0
-                    then
-                      Just <$> Parser.take lenNextCol
-                    else pure Nothing
-                case decode nextColBs of
-                  Right Nothing -> fail "Got SQL NULL but no nulls accepted" -- TODO: Please improve the failure message.. show field name and target Haskell type if we can
-                  Right (Just v) -> pure v
-                  Left err -> fail err
+                if lenNextCol >= 0
+                  then do
+                    nextColBs <- Parser.take lenNextCol
+                    case decode nextColBs of
+                      Right v -> pure v
+                      Left err -> fail err
+                  else case fdec.decodesSqlNullTo of
+                    Right v -> pure v
+                    Left err -> fail err
         _ -> error "singleField expected a single column OID but got 0 or >1",
       rowColumnsTypeCheck = \case
-        [singleColInfo] -> [(singleColInfo, allowedPgTypes singleColInfo)]
+        [singleColInfo] -> [(singleColInfo, fdec.allowedPgTypes singleColInfo)]
         _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
       numExpectedColumns = 1
     }
@@ -244,11 +246,13 @@ class FromPgRow a where
 compositeTypeDecoder :: forall a. RowDecoder a -> FieldDecoder a
 compositeTypeDecoder (RowDecoder {..}) =
   FieldDecoder
-    { fieldValueDecoder = \compositeTypeOid -> \case
-        Nothing -> Right Nothing -- Left "Got NULL in composite type but it was not allowed"
-        Just bs -> case Parser.parseOnly (parserForRecord compositeTypeOid.encodingContext <* Parser.endOfInput "compositeTypeDecoder") bs of
-          Parser.ParseOk v -> Right (Just v)
-          Parser.ParseFail err -> Left err,
+    { fieldValueDecoder = \compositeTypeOid ->
+        let prs = parserForRecord compositeTypeOid.encodingContext <* Parser.endOfInput
+         in \bs ->
+              case Parser.parseOnly prs bs of
+                Parser.ParseOk v -> Right v
+                Parser.ParseFail err -> Left err,
+      decodesSqlNullTo = Left "TODO: composeTypeDecoder decodesSqlNullTo",
       allowedPgTypes = const True -- There's no way to enforce a custom type's OID. We only check if it's structurally the same in the parser (same subtypes in same order)
     }
   where
@@ -266,7 +270,7 @@ compositeTypeDecoder (RowDecoder {..}) =
         pure (oid, sizeBs <> bs)
       let typecheckedCols = rowColumnsTypeCheck (map (mkColInfo . fst) cols)
       unless (all snd typecheckedCols) $ fail $ "Parser for composite found type OIDs " ++ show (map fst cols) ++ " but expected different"
-      case Parser.parseOnly (fullRowDecoder (map (mkColInfo . fst) cols) <* Parser.endOfInput "parserForRecord") (mconcat $ map snd cols) of
+      case Parser.parseOnly (fullRowDecoder (map (mkColInfo . fst) cols) <* Parser.endOfInput) (mconcat $ map snd cols) of
         Parser.ParseOk v -> pure v
         Parser.ParseFail err -> error $ "Error decoding composite type: " ++ show err
 
@@ -783,10 +787,11 @@ binaryFloat4Decoder = castWord32ToFloat . either error id . BinSer.decodeWord32B
 binaryFloat8Decoder :: ByteString -> Double
 binaryFloat8Decoder = castWord64ToDouble . either error id . BinSer.decodeWord64BE 0
 
-parsePgType :: [Oid] -> (Maybe ByteString -> Either String (Maybe a)) -> FieldDecoder a
-parsePgType !requiredTypeOids !fieldValueDecoder =
+parsePgType :: String -> [Oid] -> (ByteString -> Either String a) -> FieldDecoder a
+parsePgType !typeName !requiredTypeOids !fieldValueDecoder =
   FieldDecoder
     { fieldValueDecoder = \_oid -> fieldValueDecoder,
+      decodesSqlNullTo = Left $ "Cannot decode SQL null as the Haskell " ++ typeName ++ " type. Use a `Maybe " ++ show typeName ++ "`",
       allowedPgTypes = (`elem` requiredTypeOids) . fieldTypeOid
     }
 
@@ -794,9 +799,9 @@ instance FromPgField () where
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \_oid -> \case
-          Just "" -> Right (Just ())
-          Just bs -> Left $ "Invalid value '" ++ show bs ++ "' for postgres void type"
-          Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell () type. Use a `Maybe ()`",
+          "" -> Right ()
+          bs -> Left $ "Invalid value '" ++ show bs ++ "' for postgres void type",
+        decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell () type. Use a `Maybe ()`",
         allowedPgTypes = (== voidOid) . fieldTypeOid
       }
 
@@ -819,9 +824,8 @@ instance FromPgField Int where
     FieldDecoder
       { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
           let !decode = binaryIntDecoder oid
-           in \case
-                Just bs -> Just <$> decode bs
-                Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`",
+           in \bs -> decode bs,
+        decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`",
         allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
@@ -829,14 +833,31 @@ instance FromPgField Int where
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Int" intRowDecoder
 
+-- instance {-# OVERLAPPING #-} FromPgField (Maybe Int) where
+--   -- This overlapping instance isn't pretty, but it reduces memory
+--   -- usage and improves performance a bit
+--   fieldDecoder = error "TODO Maybe Int"
+
+--   -- FieldDecoder
+--   --   { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
+--   --       let !decode = binaryIntDecoder oid
+--   --        in \case
+--   --             Just bs -> Just <$> decode bs
+--   --             Nothing -> Right Nothing,
+--   --     allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
+--   --   }
+--   {-# NOINLINE singleFieldRowDecoder #-}
+--   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+--   {-# INLINE inlinedSingleFieldRowDecoder #-}
+--   inlinedSingleFieldRowDecoder = intRowDecoder
+
 instance FromPgField Int16 where
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder =
           let !decode = binaryIntDecoder int2Oid
-           in const $ \case
-                Just bs -> Just <$> decode bs
-                Nothing -> Right Nothing, -- Left "Cannot decode SQL null as the Haskell Int16 type. Use a `Maybe Int16`",
+           in const decode,
+        decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Int16 type. Use a `Maybe Int16`",
         allowedPgTypes = (== int2Oid) . fieldTypeOid
       }
 
@@ -854,11 +875,8 @@ int32RowDecoder =
 instance FromPgField Int32 where
   fieldDecoder =
     FieldDecoder
-      { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
-          let !decode = binaryIntDecoder oid
-           in \case
-                Just bs -> Just <$> decode bs
-                Nothing -> Right Nothing, -- Left "Cannot decode SQL null as the Haskell Int32 type. Use a `Maybe Int32`",
+      { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} -> binaryIntDecoder oid,
+        decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Int32 type. Use a `Maybe Int32`",
         allowedPgTypes = (`elem` [int2Oid, int4Oid]) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
@@ -881,11 +899,8 @@ int64RowDecoder =
 instance FromPgField Int64 where
   fieldDecoder =
     FieldDecoder
-      { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
-          let !decode = binaryIntDecoder oid
-           in \case
-                Just bs -> Just <$> decode bs
-                Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell Int64 type. Use a `Maybe Int64`",
+      { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} -> binaryIntDecoder oid,
+        decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Int64 type. Use a `Maybe Int64`",
         allowedPgTypes = (`elem` [int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
@@ -898,15 +913,14 @@ instance FromPgField Integer where
     FieldDecoder
       { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
           let !decodeInt = binaryIntDecoder @Int64 oid
-           in \case
-                Just bs
-                  | oid /= numericOid -> Just . fromIntegral <$> decodeInt bs
-                  | otherwise -> case Parser.parseOnly (scientificDecoder True <* Parser.endOfInput "Integer") bs of
-                      Parser.ParseOk sci -> case floatingOrInteger @Double @Integer sci of
-                        Right i -> Right (Just i)
-                        Left _ -> Left "Internal error in Hpgsql. Scientific to Integer conversion failed"
-                      Parser.ParseFail err -> Left err
-                Nothing -> Right Nothing, -- Left "Cannot decode SQL null as the Haskell Integer type. Use a `Maybe Integer`",
+           in if oid /= numericOid
+                then fmap fromIntegral <$> decodeInt
+                else \bs -> case Parser.parseOnly (scientificDecoder True <* Parser.endOfInput) bs of
+                  Parser.ParseOk sci -> case floatingOrInteger @Double @Integer sci of
+                    Right i -> Right i
+                    Left _ -> Left "Internal error in Hpgsql. Scientific to Integer conversion failed"
+                  Parser.ParseFail err -> Left err,
+        decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Integer type. Use a `Maybe Integer`",
         allowedPgTypes = (`elem` [int8Oid, numericOid, int4Oid, int2Oid]) . fieldTypeOid
       }
 
@@ -915,8 +929,8 @@ instance FromPgField Oid where
     FieldDecoder
       { fieldValueDecoder = \_ -> \case
           -- Oids are just int4
-          Just bs -> Just . Oid <$> binaryIntDecoder int4Oid bs
-          Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell Oid type. Use a `Maybe Oid`",
+          bs -> Oid <$> binaryIntDecoder int4Oid bs,
+        decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Oid type. Use a `Maybe Oid`",
         allowedPgTypes = (== oidOid) . fieldTypeOid
       }
 
@@ -926,14 +940,21 @@ floatRowDecoder =
   inlinableRowDecoder [float4Oid] Parser.takeFloatBEWithFieldLength
 
 instance FromPgField Float where
-  fieldDecoder = parsePgType [float4Oid] $ \case
-    Just bs -> Right $ Just $ binaryFloat4Decoder bs
-    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell Float type. Use a `Maybe Float`"
+  fieldDecoder = parsePgType "Float" [float4Oid] $ Right . binaryFloat4Decoder
   {-# NOINLINE singleFieldRowDecoder #-}
   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Float" floatRowDecoder
 
+-- instance {-# OVERLAPPING #-} FromPgField (Maybe Float) where
+--   -- This overlapping instance isn't pretty, but it reduces memory
+--   -- usage and improves performance a bit
+--   fieldDecoder = error "TODO Maybe Float"
+--   {-# NOINLINE singleFieldRowDecoder #-}
+--   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+--   {-# INLINE inlinedSingleFieldRowDecoder #-}
+--   inlinedSingleFieldRowDecoder = floatRowDecoder
+
 {-# INLINE doubleRowDecoder #-}
 doubleRowDecoder :: RowDecoder (Maybe Double)
 doubleRowDecoder =
@@ -949,12 +970,11 @@ instance FromPgField Double where
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
-          let !decoder
+          let decoder
                 | oid == float8Oid = binaryFloat8Decoder
                 | otherwise = float2Double . binaryFloat4Decoder
-           in \case
-                Just bs -> Right $ Just $ decoder bs
-                Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`",
+           in Right . decoder,
+        decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`",
         allowedPgTypes = (`elem` [float8Oid, float4Oid]) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
@@ -962,6 +982,15 @@ instance FromPgField Double where
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Double" doubleRowDecoder
 
+-- instance {-# OVERLAPPING #-} FromPgField (Maybe Double) where
+--   -- This overlapping instance isn't pretty, but it reduces memory
+--   -- usage and improves performance a bit
+--   fieldDecoder = error "TODO Maybe Double"
+--   {-# NOINLINE singleFieldRowDecoder #-}
+--   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+--   {-# INLINE inlinedSingleFieldRowDecoder #-}
+--   inlinedSingleFieldRowDecoder = doubleRowDecoder
+
 -- | Allows you to specify a type (and other checks, possibly) for a `FieldDecoder`.
 -- This can be useful to ensure you're not accidentally decoding a different type.
 --
@@ -1019,17 +1048,15 @@ instance FromPgField Scientific where
           if fieldTypeOid /= numericOid
             then
               let intdec = binaryIntDecoder @Int64 fieldTypeOid
-               in \case
-                    Just bs -> Just . flip scientific 0 . fromIntegral <$> intdec bs
-                    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`"
+               in \bs -> flip scientific 0 . fromIntegral <$> intdec bs
             else \case
-              Just bs ->
+              bs ->
                 -- TODO: There is loss converting from Float/Double to Scientific, but it might be quite small, so should we accept
                 -- float4Oid and float8Oid here?
-                case Parser.parseOnly (scientificDecoder False <* Parser.endOfInput "Scientific") bs of
-                  Parser.ParseOk sci -> Right (Just sci)
-                  Parser.ParseFail err -> Left err
-              Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`",
+                case Parser.parseOnly (scientificDecoder False <* Parser.endOfInput) bs of
+                  Parser.ParseOk sci -> Right sci
+                  Parser.ParseFail err -> Left err,
+        decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`",
         allowedPgTypes = (`elem` [numericOid, int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
   {-# NOINLINE singleFieldRowDecoder #-}
@@ -1060,40 +1087,45 @@ boolRowDecoder =
   inlinableRowDecoder [boolOid] $ fmap (== 1) <$> Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1
 
 instance FromPgField Bool where
-  fieldDecoder = parsePgType [boolOid] $ \case
-    Just bs -> Right $ Just $ bs == binaryTrue
-    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell Bool type. Use a `Maybe Bool`"
+  fieldDecoder = parsePgType "Bool" [boolOid] $ \bs -> Right $ bs == binaryTrue
   {-# NOINLINE singleFieldRowDecoder #-}
   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Bool" boolRowDecoder
 
+-- instance {-# OVERLAPPING #-} FromPgField (Maybe Bool) where
+--   -- This overlapping instance isn't pretty, but it reduces memory
+--   -- usage and improves performance a bit
+--   fieldDecoder = error "TODO Maybe Bool"
+--   {-# NOINLINE singleFieldRowDecoder #-}
+--   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+--   {-# INLINE inlinedSingleFieldRowDecoder #-}
+--   inlinedSingleFieldRowDecoder = boolRowDecoder
+
 instance FromPgField Char where
   fieldDecoder =
     let textParser = fieldValueDecoder (fieldDecoder @Text)
      in FieldDecoder
           { fieldValueDecoder = \colInfo@FieldInfo {fieldTypeOid = oid} ->
               let !decodeText = textParser colInfo
-               in \mbs -> case mbs of
-                    Just bs ->
-                      if oid == charOid
-                        -- TODO: Postgres has values of type "char" in the pg_type.typcategory table.
-                        -- We should test this instance works with those, and we haven't yet.
-                        then Right $ Just $ BSC.head bs
-                        else case decodeText mbs of
-                          Left err -> Left err
-                          Right (Just t) -> if Text.length t > 1 then Left "Cannot parse text with more than one character into a Haskell Char type." else Right (Just $ Text.head t)
-                          Right Nothing -> Right Nothing
-                    Nothing -> Right Nothing, -- Left "Cannot decode SQL null as the Haskell Char type. Use a `Maybe Char`",
-                    -- TODO: All the varchar types?
+               in \bs ->
+                    if oid == charOid
+                      -- TODO: Postgres has values of type "char" in the pg_type.typcategory table.
+                      -- We should test this instance works with those, and we haven't yet.
+                      then Right $ BSC.head bs
+                      else case decodeText bs of
+                        Left err -> Left err
+                        Right t -> if Text.length t > 1 then Left "Cannot parse text with more than one character into a Haskell Char type." else Right (Text.head t),
+            decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Char type. Use a `Maybe Char`",
+            -- TODO: All the varchar types?
             allowedPgTypes = (`elem` [charOid, textOid]) . fieldTypeOid
           }
 
 instance FromPgField ByteString where
-  fieldDecoder = parsePgType [byteaOid] Right
+  fieldDecoder = parsePgType "byteString" [byteaOid] Right
 
 instance FromPgField LBS.ByteString where
-  fieldDecoder = parsePgType [byteaOid] $ Right . fmap LBS.fromStrict
+  fieldDecoder = parsePgType "ByteString" [byteaOid] $ Right . LBS.fromStrict
 
 {-# INLINE textDecoder #-}
 textDecoder :: RowDecoder (Maybe Text)
@@ -1106,27 +1138,29 @@ textDecoder =
       else pure Nothing
 
 instance FromPgField Text where
-  fieldDecoder = parsePgType [textOid, varcharOid, nameOid] $ \case
-    Just bs -> Right $ Just $ decodeUtf8 bs
-    -- TODO: Use some faster unsafeDecodeUtf8 function?
-    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell Text type. Use a `Maybe Text`"
+  -- TODO: Use some faster unsafeDecodeUtf8 function?
+  fieldDecoder = parsePgType "Text" [textOid, varcharOid, nameOid] $ \bs -> Right $ decodeUtf8 bs
   {-# NOINLINE singleFieldRowDecoder #-}
   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Text" textDecoder
 
+-- instance {-# OVERLAPPING #-} FromPgField (Maybe Text) where
+--   -- This overlapping instance isn't pretty, but it reduces memory
+--   -- usage and improves performance a bit
+--   fieldDecoder = error "TODO Maybe Text"
+--   {-# NOINLINE singleFieldRowDecoder #-}
+--   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+--   {-# INLINE inlinedSingleFieldRowDecoder #-}
+--   inlinedSingleFieldRowDecoder = textDecoder
+
 instance FromPgField LT.Text where
-  fieldDecoder = parsePgType [textOid, varcharOid, nameOid] $ \case
-    Just bs -> Right $ Just $ LT.fromStrict $ decodeUtf8 bs
-    -- TODO: Use some faster unsafeDecodeUtf8 function?
-    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell Text type. Use a `Maybe Text`"
+  -- TODO: Use some faster unsafeDecodeUtf8 function?
+  fieldDecoder = parsePgType "Text" [textOid, varcharOid, nameOid] $ \bs -> Right $ LT.fromStrict $ decodeUtf8 bs
 
 instance FromPgField String where
-  fieldDecoder = parsePgType [textOid, varcharOid, nameOid] $ \case
-    -- connection option).
-    Just bs -> Right $ Just $ Text.unpack $ decodeUtf8 bs
-    -- TODO: Use some faster unsafeDecodeUtf8 function?
-    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell String type. Use a `Maybe String`"
+  -- TODO: Use some faster unsafeDecodeUtf8 function?
+  fieldDecoder = parsePgType "String" [textOid, varcharOid, nameOid] $ \bs -> Right $ Text.unpack $ decodeUtf8 bs
 
 -- | This instance does not work if you have fillTypeInfoCache disabled (that would be a non-default
 -- connection option).
@@ -1157,78 +1191,81 @@ utcTimeRowDecoder =
       _ -> pure Nothing
 
 instance FromPgField UTCTime where
-  fieldDecoder = parsePgType [timestamptzOid] $ \case
-    Just bs -> do
+  fieldDecoder = parsePgType "UTCTime" [timestamptzOid] $ \case
+    bs -> do
       -- See https://github.com/postgres/postgres/blob/50cb7505b3010736b9a7922e903931534785f3aa/src/backend/utils/adt/timestamp.c#L1909
       totalusecs <- BinSer.decodeInt64BE 0 bs
       let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
           parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-      Right $ Just $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell UTCTime type. Use a `Maybe UTCTime`"
+      Right $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
   {-# NOINLINE singleFieldRowDecoder #-}
   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# NOINLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "UTCTime" utcTimeRowDecoder
 
+-- instance {-# OVERLAPPING #-} FromPgField (Maybe UTCTime) where
+--   -- This overlapping instance isn't pretty, but it reduces memory
+--   -- usage and improves performance a bit
+--   fieldDecoder = error "TODO Maybe UTCTime"
+--   {-# NOINLINE singleFieldRowDecoder #-}
+--   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+--   {-# INLINE inlinedSingleFieldRowDecoder #-}
+--   inlinedSingleFieldRowDecoder = utcTimeRowDecoder
+
 instance FromPgField (Unbounded UTCTime) where
-  fieldDecoder = parsePgType [timestamptzOid] $ \case
-    Just bs -> do
+  fieldDecoder = parsePgType "Unbounded UTCTime" [timestamptzOid] $ \case
+    bs -> do
       -- See https://github.com/postgres/postgres/blob/50cb7505b3010736b9a7922e903931534785f3aa/src/backend/utils/adt/timestamp.c#L1909
       totalusecs <- BinSer.decodeInt64BE 0 bs
       Right $
         if totalusecs == minBound
-          then Just NegInfinity
+          then NegInfinity
           else
             if totalusecs == maxBound
-              then Just PosInfinity
+              then PosInfinity
               else
                 let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
                     parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-                 in Just $ Finite $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell (Unbounded UTCTime) type. Use a `Maybe (Unbounded UTCTime)`"
+                 in Finite $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
 
 instance FromPgField ZonedTime where
-  fieldDecoder = parsePgType [timestamptzOid] $ \case
-    Just bs -> do
+  fieldDecoder = parsePgType "ZonedTime" [timestamptzOid] $ \case
+    bs -> do
       -- See https://github.com/postgres/postgres/blob/50cb7505b3010736b9a7922e903931534785f3aa/src/backend/utils/adt/timestamp.c#L1909
       totalusecs <- BinSer.decodeInt64BE 0 bs
       let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
           parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-      Right $ Just $ utcToZonedTime utc $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell ZonedTime type. Use a `Maybe ZonedTime`"
+      Right $ utcToZonedTime utc $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
 
 instance FromPgField (Unbounded ZonedTime) where
-  fieldDecoder = parsePgType [timestamptzOid] $ \case
-    Just bs -> do
+  fieldDecoder = parsePgType "Unbounded ZonedTime" [timestamptzOid] $ \case
+    bs -> do
       -- See https://github.com/postgres/postgres/blob/50cb7505b3010736b9a7922e903931534785f3aa/src/backend/utils/adt/timestamp.c#L1909
       totalusecs <- BinSer.decodeInt64BE 0 bs
       Right $
         if totalusecs == minBound
-          then Just NegInfinity
+          then NegInfinity
           else
             if totalusecs == maxBound
-              then Just PosInfinity
+              then PosInfinity
               else
                 let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
                     parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-                 in Just $ Finite $ utcToZonedTime utc $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell ZonedTime type. Use a `Maybe ZonedTime`"
+                 in Finite $ utcToZonedTime utc $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
 
 instance FromPgField LocalTime where
-  fieldDecoder = parsePgType [timestampOid] $ \case
-    Just bs -> do
+  fieldDecoder = parsePgType "LocalTime" [timestampOid] $ \case
+    bs -> do
       totalusecs <- BinSer.decodeInt64BE 0 bs
       let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
           parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-      Right $ Just $ LocalTime parsedDate (timeToTimeOfDay $ picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell LocalTime type. Use a `Maybe LocalTime`"
+      Right $ LocalTime parsedDate (timeToTimeOfDay $ picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
 
 instance FromPgField TimeOfDay where
-  fieldDecoder = parsePgType [timeOid] $ \case
-    Just bs -> do
+  fieldDecoder = parsePgType "TimeOfDay" [timeOid] $ \case
+    bs -> do
       usecs <- BinSer.decodeInt64BE 0 bs
-      Right $ Just $ timeToTimeOfDay $ picosecondsToDiffTime $ fromIntegral usecs * 1_000_000
-    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell TimeOfDay type. Use a `Maybe TimeOfDay`"
+      Right $ timeToTimeOfDay $ picosecondsToDiffTime $ fromIntegral usecs * 1_000_000
 
 {-# INLINE dayRowDecoder #-}
 dayRowDecoder :: RowDecoder (Maybe Day)
@@ -1237,51 +1274,55 @@ dayRowDecoder =
    in inlinableRowDecoder [dateOid] $ fmap int32ToDay <$> Parser.takeInt32BEWithFieldLength
 
 instance FromPgField Day where
-  fieldDecoder = parsePgType [dateOid] $ \case
-    Just bs -> do
+  fieldDecoder = parsePgType "Day" [dateOid] $ \case
+    bs -> do
       -- There is a very specific conversion function for these, which I poorly translated to Haskell
       -- https://github.com/postgres/postgres/blob/799959dc7cf0e2462601bea8d07b6edec3fa0c4f/src/backend/utils/adt/datetime.c#L321
       -- But I found a simpler way to do this. Let's see if it works in our property based tests
       jd <- BinSer.decodeInt32BE 0 bs
-      Right $ Just $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
-    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell Day type. Use a `Maybe Day`"
+      Right $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
   {-# NOINLINE singleFieldRowDecoder #-}
   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Day" dayRowDecoder
 
+-- instance {-# OVERLAPPING #-} FromPgField (Maybe Day) where
+--   -- This overlapping instance isn't pretty, but it reduces memory
+--   -- usage and improves performance a bit
+--   fieldDecoder = error "TODO Maybe Day"
+--   {-# NOINLINE singleFieldRowDecoder #-}
+--   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+--   {-# INLINE inlinedSingleFieldRowDecoder #-}
+--   inlinedSingleFieldRowDecoder = dayRowDecoder
+
 instance FromPgField (Unbounded Day) where
-  fieldDecoder = parsePgType [dateOid] $ \case
-    Just bs -> do
+  fieldDecoder = parsePgType "Unbounded Day" [dateOid] $ \case
+    bs -> do
       -- There is a very specific conversion function for these, which I poorly translated to Haskell
       -- https://github.com/postgres/postgres/blob/799959dc7cf0e2462601bea8d07b6edec3fa0c4f/src/backend/utils/adt/datetime.c#L321
       -- But I found a simpler way to do this. Let's see if it works in our property based tests
       jd <- BinSer.decodeInt32BE 0 bs
       Right $
         if jd == minBound
-          then Just NegInfinity
+          then NegInfinity
           else
             if jd == maxBound
-              then Just PosInfinity
+              then PosInfinity
               else
-                Just $ Finite $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
-    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell (Unbounded Day) type. Use a `Maybe (Unbounded Day)`"
+                Finite $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
 
 instance FromPgField CalendarDiffTime where
-  fieldDecoder = parsePgType [intervalOid] $ \case
-    Just bs -> do
-      nMicrosecs <- BinSer.decodeInt64BE 0 bs
-      nDays <- BinSer.decodeInt32BE 8 bs
-      nMonths <- BinSer.decodeInt32BE 12 bs
-      Right $ Just $ CalendarDiffTime {ctMonths = fromIntegral nMonths, ctTime = secondsToNominalDiffTime (fromIntegral nDays * 86400) + realToFrac (picosecondsToDiffTime (fromIntegral nMicrosecs * 1_000_000))}
-    Nothing -> pure Nothing -- Left "Cannot decode SQL null as the Haskell CalendarDiffTime type. Use a `Maybe CalendarDiffTime`"
+  fieldDecoder = parsePgType "CalendarDiffTime " [intervalOid] $ \bs -> do
+    nMicrosecs <- BinSer.decodeInt64BE 0 bs
+    nDays <- BinSer.decodeInt32BE 8 bs
+    nMonths <- BinSer.decodeInt32BE 12 bs
+    Right $ CalendarDiffTime {ctMonths = fromIntegral nMonths, ctTime = secondsToNominalDiffTime (fromIntegral nDays * 86400) + realToFrac (picosecondsToDiffTime (fromIntegral nMicrosecs * 1_000_000))}
 
 instance FromPgField UUID where
-  fieldDecoder = parsePgType [uuidOid] $ \case
-    Just bs -> case UUID.fromByteString (LBS.fromStrict bs) of
-      Just uuid -> Right (Just uuid)
+  fieldDecoder = parsePgType "UUID" [uuidOid] $ \case
+    bs -> case UUID.fromByteString (LBS.fromStrict bs) of
+      Just uuid -> Right uuid
       Nothing -> Left "Bug in Hpgsql: UUID field could not be decoded"
-    Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell UUID type. Use a `Maybe UUID`"
 
 instance FromPgField Aeson.Value where
   fieldDecoder =
@@ -1289,14 +1330,12 @@ instance FromPgField Aeson.Value where
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
             let -- jsonb has a byte prepended to the contents and json does not
-                -- TODO: Does `BS.drop 1` get inlined into `Aeson.decodeStrict` further down?
-                -- Might be an interesting case study
                 !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
              in \case
-                  Just bs -> case Aeson.decodeStrict @Aeson.Value $ fixJsonb bs of
-                    Just d -> Right (Just d)
-                    Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid."
-                  Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
+                  bs -> case Aeson.decodeStrict $ fixJsonb bs of
+                    Just d -> Right d
+                    Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid.",
+        decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
 
@@ -1306,17 +1345,9 @@ nullableField :: FieldDecoder a -> FieldDecoder (Maybe a)
 nullableField FieldDecoder {..} =
   FieldDecoder
     { fieldValueDecoder = \oid ->
-        let !origFieldValueParser = fieldValueDecoder oid
-         in \case
-              -- TODO: we have multiple layers of Maybe/NULL here.
-              -- What should we do? If the supplied decoder returns
-              -- Nothing, do we return `Just Nothing`? Or do we follow
-              -- SQL's viral NULL semantics?
-              -- Think about a custom user type that decodes NULL into a Just,
-              -- and test that with all decoding combinations we provide (
-              -- singleField, nullableField, specialized inlined/not-inlined row decoders, etc.)
-              Nothing -> Right Nothing
-              justBs -> Just <$> origFieldValueParser justBs,
+        let origFieldValueParser = fieldValueDecoder oid
+         in \bs -> Just <$> origFieldValueParser bs,
+      decodesSqlNullTo = Right Nothing,
       allowedPgTypes
     }
 
@@ -1332,30 +1363,40 @@ nonNullableRowDec haskellTypeName rdec =
           numExpectedColumns = rdec.numExpectedColumns
         }
 
-{-# INLINE nullableRowDec #-}
-nullableRowDec :: RowDecoder a -> RowDecoder (Maybe a)
-nullableRowDec rdec =
-  RowDecoder
-    { fullRowDecoder = \finfos -> do
-        -- Peek the field length, but only consume it
-        -- if we get a NULL. Urgh.
-        fieldLen <- Parser.peekInt32BE
-        if fieldLen == (-1)
-          then do
-            Parser.skip 4
-            pure Nothing
-          else
-            Just <$> rdec.fullRowDecoder finfos,
-      rowColumnsTypeCheck = rdec.rowColumnsTypeCheck,
-      numExpectedColumns = rdec.numExpectedColumns
-    }
-
 instance (FromPgField a) => FromPgField (Maybe a) where
   fieldDecoder = nullableField fieldDecoder
+
   {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = nullableRowDec inlinedSingleFieldRowDecoder
+  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = nullableRowDec inlinedSingleFieldRowDecoder
+  inlinedSingleFieldRowDecoder =
+    RowDecoder
+      { fullRowDecoder = \finfos ->
+          let frd = inlinedSingleFieldRowDecoder.fullRowDecoder finfos
+           in do
+                -- TODO: We're decoding the field length twice with
+                -- the peek call when the value isn't NULL.
+                -- Maybe we should make `FromPgField`'s new methods
+                -- be two `Parser` objects: one for both length and field
+                -- and another only for the field (but how would that work
+                -- without the length..? It wouldn't.)
+                -- Maybe we do the `Parser (Maybe a)` for `a` types, then.
+                -- We can build a `Parser a` from that with `decodesSqlNullTo`
+                -- and with inlining there's nothing to lose?
+                fieldLen <- Parser.peekInt32BE
+                if fieldLen == (-1)
+                  then case fieldDecoder.decodesSqlNullTo of
+                    Left err -> fail err
+                    Right v -> Parser.skip 4 >> pure v
+                  else do
+                    Just <$> frd,
+        rowColumnsTypeCheck =
+          let fdec = fieldDecoder @a
+           in \case
+                [singleColInfo] -> [(singleColInfo, fdec.allowedPgTypes singleColInfo)]
+                _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+        numExpectedColumns = 1
+      }
 
 allowOnlyArrayTypes :: FieldInfo -> Bool
 allowOnlyArrayTypes fieldInfo =
@@ -1373,12 +1414,12 @@ instance {-# OVERLAPPING #-} forall a. (FromPgField a) => FromPgField (Vector (V
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \colInfo ->
-          let !arrayFieldDecoder = arrayParser colInfo.encodingContext <* Parser.endOfInput "Vector (Vector a)"
+          let !arrayFieldDecoder = arrayParser colInfo.encodingContext <* Parser.endOfInput
            in \case
-                Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell (Vector (Vector a)) type. Use a `Maybe (Vector (Vector a))`"
-                Just bs -> case Parser.parseOnly arrayFieldDecoder bs of
-                  Parser.ParseOk v -> Right (Just v)
+                bs -> case Parser.parseOnly arrayFieldDecoder bs of
+                  Parser.ParseOk v -> Right v
                   Parser.ParseFail err -> Left err,
+        decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell (Vector (Vector a)) type. Use a `Maybe (Vector (Vector a))`",
         allowedPgTypes = allowOnlyArrayTypes
       }
     where
@@ -1404,11 +1445,15 @@ instance {-# OVERLAPPING #-} forall a. (FromPgField a) => FromPgField (Vector (V
           Vector.replicateM lengthEachRow $
             do
               size :: Int <- fromIntegral <$> Parser.takeInt32BE
-              elementBs <- if size == (-1) then pure Nothing else Just <$> Parser.take size
-              case elementParser.fieldValueDecoder elementColInfo elementBs of
-                Left err -> fail $ "Error parsing array element: " ++ show err
-                Right (Just el) -> pure el
-                Right Nothing -> fail "Found an array element that is NULL" -- TODO: I think this is a bug of ours, because what if a ~ Maybe b?
+              if size == (-1)
+                then case elementParser.decodesSqlNullTo of
+                  Left err -> fail err
+                  Right v -> pure v
+                else do
+                  elementBs <- Parser.take size
+                  case elementParser.fieldValueDecoder elementColInfo elementBs of
+                    Left err -> fail $ "Error parsing array element: " ++ show err
+                    Right el -> pure el
 
 {-# INLINE genericFromPgRow #-}
 
@@ -1540,7 +1585,9 @@ untypedFieldEncoder enc = FieldEncoder {toTypeOid = \_ -> Nothing, toPgField = e
 rawBytesFieldDecoder :: FieldDecoder ByteString
 rawBytesFieldDecoder =
   FieldDecoder
-    { fieldValueDecoder = const Right,
+    { fieldValueDecoder = \_oid -> \case
+        bs -> Right bs,
+      decodesSqlNullTo = Left "Cannot decode SQL null as the `rawBytesFieldDecoder`.",
       allowedPgTypes = const True
     }
 
@@ -1568,12 +1615,12 @@ arrayField !replicateFunction !elementParser =
   -- From https://github.com/postgres/postgres/blob/5941946d0934b9eccb0d5bfebd40b155249a0130/src/backend/utils/adt/arrayfuncs.c#L1548
   FieldDecoder
     { fieldValueDecoder = \colInfo ->
-        let !arrayFieldDecoder = arrayParser colInfo.encodingContext <* Parser.endOfInput "arrayField"
+        let !arrayFieldDecoder = arrayParser colInfo.encodingContext <* Parser.endOfInput
          in \case
-              Nothing -> Right Nothing -- Left "Cannot decode SQL null as the Haskell Vector type. Use a `Maybe (Vector a)`"
-              Just bs -> case Parser.parseOnly arrayFieldDecoder bs of
-                Parser.ParseOk v -> Right (Just v)
+              bs -> case Parser.parseOnly arrayFieldDecoder bs of
+                Parser.ParseOk v -> Right v
                 Parser.ParseFail err -> Left err,
+      decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Vector type. Use a `Maybe (Vector a)`",
       allowedPgTypes = allowOnlyArrayTypes
     }
   where
@@ -1592,8 +1639,12 @@ arrayField !replicateFunction !elementParser =
           unless (elementParser.allowedPgTypes elementColInfo) $ fail $ "Array contains elements of type OID " ++ show elementTypeOid ++ " but decoder does not handle that type"
           replicateFunction dim_i $ do
             size :: Int <- fromIntegral <$> Parser.takeInt32BE
-            elementBs <- if size == (-1) then pure Nothing else Just <$> Parser.take size
-            case elementParser.fieldValueDecoder elementColInfo elementBs of
-              Left err -> fail $ "Error parsing array element: " ++ show err
-              Right (Just el) -> pure el
-              Right Nothing -> fail "Found an array element that is NULL" -- TODO: I think this is a bug of ours, because what if a ~ Maybe b?
+            if size == (-1)
+              then case elementParser.decodesSqlNullTo of
+                Left err -> fail err
+                Right v -> pure v
+              else do
+                elementBs <- Parser.take size
+                case elementParser.fieldValueDecoder elementColInfo elementBs of
+                  Left err -> fail $ "Error parsing array element: " ++ show err
+                  Right el -> pure el
diff --git a/hpgsql/src/Hpgsql/Internal.hs b/hpgsql/src/Hpgsql/Internal.hs
index 4cfa3c0..8c427fe 100644
--- a/hpgsql/src/Hpgsql/Internal.hs
+++ b/hpgsql/src/Hpgsql/Internal.hs
@@ -1418,7 +1418,7 @@ consumeStreamingResults rp conn qryId = S.effect $ do
           S.concat $
             S.mapM
               ( \(DataRows rowColumnData) ->
-                  case Parser.parseOnly (Parser.parseMany rowparser <* Parser.endOfInput "DataRows") rowColumnData of
+                  case Parser.parseOnly (Parser.parseMany rowparser <* Parser.endOfInput) rowColumnData of
                     Parser.ParseOk rows -> pure rows
                     Parser.ParseFail err -> throwIrrecoverableErrorWithStatement qText $ "Failed parsing a row: " <> Text.pack (show err)
               )
diff --git a/hpgsql/src/Hpgsql/SimpleParser.hs b/hpgsql/src/Hpgsql/SimpleParser.hs
index 28fec4d..5c9e499 100644
--- a/hpgsql/src/Hpgsql/SimpleParser.hs
+++ b/hpgsql/src/Hpgsql/SimpleParser.hs
@@ -249,9 +249,9 @@ parseManyRows = Parser $ \idx' bs' _kf ks -> let restIdx = go idx' bs' in ks res
 {-# INLINE parseManyRows #-}
 
 -- | Succeeds only when the input has been fully consumed.
-endOfInput :: String -> Parser ()
-endOfInput debugFail = Parser $ \idx bs kf ks ->
-  if BS.length bs <= idx.idx then ks () idx bs else kf $ "endOfInput: input remaining (" ++ debugFail ++ ")"
+endOfInput :: Parser ()
+endOfInput = Parser $ \idx bs kf ks ->
+  if BS.length bs <= idx.idx then ks () idx bs else kf "endOfInput: input remaining"
 {-# INLINE endOfInput #-}
 
 -- | Run a parser and additionally return the slice of input it consumed.
diff --git a/hpgsql/src/Hpgsql/Types.hs b/hpgsql/src/Hpgsql/Types.hs
index ad19e16..8cc68ce 100644
--- a/hpgsql/src/Hpgsql/Types.hs
+++ b/hpgsql/src/Hpgsql/Types.hs
@@ -89,9 +89,8 @@ instance FromPgField PgJson where
           \FieldInfo {fieldTypeOid} ->
             let -- jsonb has a byte prepended to the contents and json does not
                 !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in \case
-                  Just bs -> Right $ Just $ PgJson $ fixJsonb bs
-                  Nothing -> pure Nothing, -- Left "Cannot decode SQL null as the Haskell PgJson type. Use a `Maybe PgJson` if you want SQL nulls",
+             in \bs -> Right $ PgJson $ fixJsonb bs,
+        decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell PgJson type. Use a `Maybe PgJson` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
 
@@ -109,11 +108,10 @@ instance (FromJSON a) => FromPgField (Aeson a) where
           \FieldInfo {fieldTypeOid} ->
             let -- jsonb has a byte prepended to the contents and json does not
                 !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in \case
-                  Just bs -> case Aeson.decodeStrict $ fixJsonb bs of
-                    Just v -> Right $ Just $ Aeson v
-                    Nothing -> Left "Failed to decode postgres JSON value into your `Aeson a` type. Are you sure it's proper JSON?"
-                  Nothing -> Right Nothing, -- Left "Cannot decode SQL null as a Haskell (Aeson a) type. Use a `Maybe (Aeson a)` if you want SQL nulls",
+             in \bs -> case Aeson.decodeStrict $ fixJsonb bs of
+                  Just v -> Right $ Aeson v
+                  Nothing -> Left "Failed to decode postgres JSON value into your `Aeson a` type. Are you sure it's proper JSON?",
+        decodesSqlNullTo = Left "Cannot decode SQL null as a Haskell (Aeson a) type. Use a `Maybe (Aeson a)` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
 

From 08cab31160c4e0db6c8b3e841af07f8f06ad0b70 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Tue, 18 Aug 2026 18:18:30 -0300
Subject: [PATCH 14/22] Try to improve the code, but inlining got worse

---
 hpgsql-benchmarks/src/Main.hs     |   2 +-
 hpgsql-tests/RowDecoderGhcCore.hs |   2 +-
 hpgsql/src/Hpgsql/Encoding.hs     | 256 ++++++++++++++++--------------
 3 files changed, 140 insertions(+), 120 deletions(-)

diff --git a/hpgsql-benchmarks/src/Main.hs b/hpgsql-benchmarks/src/Main.hs
index 25672a8..9150240 100644
--- a/hpgsql-benchmarks/src/Main.hs
+++ b/hpgsql-benchmarks/src/Main.hs
@@ -47,7 +47,7 @@ import Hpgsql.Connection (renderLibpqConnectionString)
 import qualified Hpgsql.Connection
 import qualified Hpgsql.Connection as Hpgsql
 import qualified Hpgsql.Copy
-import Hpgsql.Encoding (FromPgField (inlinedSingleFieldRowDecoder))
+import Hpgsql.Encoding (inlinedSingleFieldRowDecoder)
 import qualified Hpgsql.Encoding as Hpgsql
 import qualified Hpgsql.Query as Hpgsql
 import qualified Hpgsql.Types as Hpgsql
diff --git a/hpgsql-tests/RowDecoderGhcCore.hs b/hpgsql-tests/RowDecoderGhcCore.hs
index 512efc8..e10493c 100644
--- a/hpgsql-tests/RowDecoderGhcCore.hs
+++ b/hpgsql-tests/RowDecoderGhcCore.hs
@@ -10,7 +10,7 @@ import Data.Int (Int64)
 import Data.Text (Text)
 import Data.Time (Day, UTCTime)
 import GHC.Generics (Generic)
-import Hpgsql.Encoding (FromPgField (..), FromPgRow (..), genericFromPgRow, singleField)
+import Hpgsql.Encoding (FromPgField (..), FromPgRow (..), genericFromPgRow, inlinedSingleFieldRowDecoder, singleField)
 
 -- | BestCaseScenarioRecord's purpose is to have a very small row decoder in GHC Core
 -- for my own understanding/comprehension of what a RowDecoder gets compiled to
diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index fc9513e..6592c42 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -27,6 +27,7 @@ module Hpgsql.Encoding
     FromPgRow (..),
     RowDecoder (..), -- TODO: Can we export ctor?
     singleField,
+    singleFieldRowDecoder,
     nullableField,
     genericFromPgRow,
 
@@ -78,6 +79,7 @@ import Data.CaseInsensitive (CI)
 import qualified Data.CaseInsensitive as CI
 import Data.Coerce (coerce)
 import Data.Fixed (divMod')
+import Data.Functor ((<&>))
 import Data.Functor.Contravariant (Contravariant (..))
 import Data.Int (Int16, Int32, Int64)
 import qualified Data.List as List
@@ -159,7 +161,7 @@ instance Applicative RowDecoder where
 instance (TypeError (TypeLits.Text "RowDecoder does not have a Monad instance in Hpgsql because Hpgsql type-checks the result types of queries before having access to even the first data row. Use the Applicative class to write your instances or use the Monadic decoding variants.")) => Monad RowDecoder where
   (>>=) = error "inaccessible bind in Monad RowDecoder instance"
 
-{-# INLINE singleField #-} -- 1.2% wall time perf. gain with this
+{-# INLINE singleField #-}
 singleField :: FieldDecoder a -> RowDecoder a
 singleField fdec =
   RowDecoder
@@ -208,24 +210,63 @@ inlinableRowDecoder tyoids p =
     }
 
 class FromPgField a where
-  -- | A decoder that takes
+  {-# MINIMAL fieldDecoder #-}
   fieldDecoder :: FieldDecoder a
 
-  -- | This should be semantically equivalent to `singleField fieldDecoder`, and
-  -- it is automatically derived to be exactly that.
-  -- So as a user, you don't need to override this.
-  -- This field exists for a performance optimization within hpgsql, or for users
-  -- that really know what they're doing.
-  singleFieldRowDecoder :: RowDecoder a
-  singleFieldRowDecoder = singleField fieldDecoder
-
-  -- | This is just like `singleFieldDecoder`, but it inlines into your
-  -- `FromPgRow` instances aggressively. This will increase code size and
+  -- | This should be semantically equivalent to `singleField fieldDecoder`,
+  -- but it can be overridden (and is for base types) to a much faster implementation.
+  -- Using this when deriving your `FromPgRow` instances will increase code size and
   -- possibly compilation times somewhat, but in some cases it can make row decoders
   -- compile down to a ByteString-peeking implementation with much fewer
-  -- allocations that can be ~10% faster than the other.
+  -- allocations and thus better performance.
+  -- Any implementation of this _must_ return a `Nothing` for a SQL NULL value,
+  -- regardless of what `FieldDecoder` would do with a SQL NULL.
+  -- TODO: Move this to inside the FieldDecoder type?
+  {-# INLINE fastFieldDecoder #-}
+  fastFieldDecoder :: RowDecoder (Maybe a)
+  fastFieldDecoder =
+    let fdec = fieldDecoder
+     in RowDecoder
+          { fullRowDecoder = \case
+              [singleColInfo] -> do
+                len <- Parser.takeInt32BE
+                if len == (-1)
+                  then pure Nothing
+                  else do
+                    bs <- Parser.take (fromIntegral len)
+                    case fdec.fieldValueDecoder singleColInfo bs of
+                      Left err -> fail err
+                      Right v -> pure v
+              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+            rowColumnsTypeCheck = \case
+              [singleColInfo] -> [(singleColInfo, fdec.allowedPgTypes singleColInfo)]
+              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+            numExpectedColumns = 1
+          }
+
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder :: RowDecoder a
-  inlinedSingleFieldRowDecoder = singleFieldRowDecoder
+  inlinedSingleFieldRowDecoder =
+    let fastrdec = fastFieldDecoder @a
+     in do
+          RowDecoder
+            { fullRowDecoder = \finfos -> do
+                mv <- fastrdec.fullRowDecoder finfos
+                case mv of
+                  -- This `case` is why we require `fastFieldDecoder` to decode
+                  -- SQL NULL into `Nothing`: we do check decodesSqlNullTo.
+                  Nothing -> case fieldDecoder.decodesSqlNullTo of
+                    Left err -> fail err -- Type doesn't accept NULLs
+                    Right v -> pure v
+                  Just v -> pure v,
+              rowColumnsTypeCheck = fastrdec.rowColumnsTypeCheck,
+              numExpectedColumns = fastrdec.numExpectedColumns
+            }
+
+-- TODO: better name for `singleFieldRowDecoder`?
+{-# NOINLINE singleFieldRowDecoder #-}
+singleFieldRowDecoder :: forall a. (FromPgField a) => RowDecoder a
+singleFieldRowDecoder = inlinedSingleFieldRowDecoder
 
 class FromPgRow a where
   rowDecoder :: RowDecoder a
@@ -805,6 +846,7 @@ instance FromPgField () where
         allowedPgTypes = (== voidOid) . fieldTypeOid
       }
 
+-- TODO: Inline intRowDecoder into FromPgField? And all others too?
 {-# INLINE intRowDecoder #-}
 intRowDecoder :: RowDecoder (Maybe Int)
 intRowDecoder =
@@ -828,8 +870,8 @@ instance FromPgField Int where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`",
         allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
       }
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+  {-# INLINE fastFieldDecoder #-}
+  fastFieldDecoder = intRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Int" intRowDecoder
 
@@ -846,10 +888,8 @@ instance FromPgField Int where
 --   --             Nothing -> Right Nothing,
 --   --     allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
 --   --   }
---   {-# NOINLINE singleFieldRowDecoder #-}
---   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
---   {-# INLINE inlinedSingleFieldRowDecoder #-}
---   inlinedSingleFieldRowDecoder = intRowDecoder
+--   {-# INLINE fastFieldDecoder #-}
+--   fastFieldDecoder = intRowDecoder
 
 instance FromPgField Int16 where
   fieldDecoder =
@@ -862,14 +902,14 @@ instance FromPgField Int16 where
       }
 
 {-# INLINE int32RowDecoder #-}
-int32RowDecoder :: RowDecoder Int32
+int32RowDecoder :: RowDecoder (Maybe Int32)
 int32RowDecoder =
   inlinableRowDecoder [int2Oid, int4Oid] $ do
     fieldLen <- Parser.takeInt32BE
     case fieldLen of
-      4 -> Parser.takeInt32BE
-      (-1) -> fail "Cannot decode SQL null as the Haskell Int32 type. Use a `Maybe Int32`"
-      2 -> fromIntegral <$> Parser.takeInt16BE
+      4 -> Just <$> Parser.takeInt32BE
+      (-1) -> pure Nothing
+      2 -> Just . fromIntegral <$> Parser.takeInt16BE
       _ -> fail "Trying to decode PG int4 but it's not 2 or 4 bytes long"
 
 instance FromPgField Int32 where
@@ -879,21 +919,19 @@ instance FromPgField Int32 where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Int32 type. Use a `Maybe Int32`",
         allowedPgTypes = (`elem` [int2Oid, int4Oid]) . fieldTypeOid
       }
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = int32RowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = int32RowDecoder
+  {-# INLINE fastFieldDecoder #-}
+  fastFieldDecoder = int32RowDecoder
 
 {-# INLINE int64RowDecoder #-}
-int64RowDecoder :: RowDecoder Int64
+int64RowDecoder :: RowDecoder (Maybe Int64)
 int64RowDecoder =
   inlinableRowDecoder [int2Oid, int4Oid, int8Oid] $ do
     fieldLen <- Parser.takeInt32BE
     case fieldLen of
-      8 -> Parser.takeInt64BE
-      4 -> fromIntegral <$> Parser.takeInt32BE
-      (-1) -> fail "Cannot decode SQL null as the Haskell Int64 type. Use a `Maybe Int64`"
-      2 -> fromIntegral <$> Parser.takeInt16BE
+      8 -> Just <$> Parser.takeInt64BE
+      4 -> Just . fromIntegral <$> Parser.takeInt32BE
+      (-1) -> pure Nothing
+      2 -> Just . fromIntegral <$> Parser.takeInt16BE
       _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
 
 instance FromPgField Int64 where
@@ -903,10 +941,8 @@ instance FromPgField Int64 where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Int64 type. Use a `Maybe Int64`",
         allowedPgTypes = (`elem` [int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = int64RowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = int64RowDecoder
+  {-# INLINE fastFieldDecoder #-}
+  fastFieldDecoder = int64RowDecoder
 
 instance FromPgField Integer where
   fieldDecoder =
@@ -941,19 +977,15 @@ floatRowDecoder =
 
 instance FromPgField Float where
   fieldDecoder = parsePgType "Float" [float4Oid] $ Right . binaryFloat4Decoder
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = nonNullableRowDec "Float" floatRowDecoder
+  {-# INLINE fastFieldDecoder #-}
+  fastFieldDecoder = floatRowDecoder
 
 -- instance {-# OVERLAPPING #-} FromPgField (Maybe Float) where
 --   -- This overlapping instance isn't pretty, but it reduces memory
 --   -- usage and improves performance a bit
 --   fieldDecoder = error "TODO Maybe Float"
---   {-# NOINLINE singleFieldRowDecoder #-}
---   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
---   {-# INLINE inlinedSingleFieldRowDecoder #-}
---   inlinedSingleFieldRowDecoder = floatRowDecoder
+--   {-# INLINE fastFieldDecoder #-}
+--   fastFieldDecoder = floatRowDecoder
 
 {-# INLINE doubleRowDecoder #-}
 doubleRowDecoder :: RowDecoder (Maybe Double)
@@ -977,8 +1009,8 @@ instance FromPgField Double where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`",
         allowedPgTypes = (`elem` [float8Oid, float4Oid]) . fieldTypeOid
       }
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+  {-# INLINE fastFieldDecoder #-}
+  fastFieldDecoder = doubleRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Double" doubleRowDecoder
 
@@ -986,10 +1018,8 @@ instance FromPgField Double where
 --   -- This overlapping instance isn't pretty, but it reduces memory
 --   -- usage and improves performance a bit
 --   fieldDecoder = error "TODO Maybe Double"
---   {-# NOINLINE singleFieldRowDecoder #-}
---   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
---   {-# INLINE inlinedSingleFieldRowDecoder #-}
---   inlinedSingleFieldRowDecoder = doubleRowDecoder
+--   {-# INLINE fastFieldDecoder #-}
+--   fastFieldDecoder = doubleRowDecoder
 
 -- | Allows you to specify a type (and other checks, possibly) for a `FieldDecoder`.
 -- This can be useful to ensure you're not accidentally decoding a different type.
@@ -1033,12 +1063,12 @@ scientificDecoder mustBeInteger = do
       parseAndMult (ndigitsLeft - 1) (currexpon - 4) (val + scientific digit currexpon)
 
 {-# INLINE numericRowParser #-}
-numericRowParser :: Parser.Parser Scientific
+numericRowParser :: Parser.Parser (Maybe Scientific)
 numericRowParser = do
   fieldLen <- Parser.takeInt32BE
   case fieldLen of
-    (-1) -> fail "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`"
-    _ -> scientificDecoder False
+    (-1) -> pure Nothing
+    _ -> Just <$> scientificDecoder False
 
 instance FromPgField Scientific where
   -- See https://github.com/postgres/postgres/blob/799959dc7cf0e2462601bea8d07b6edec3fa0c4f/src/backend/utils/adt/numeric.c#L1163
@@ -1059,14 +1089,14 @@ instance FromPgField Scientific where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`",
         allowedPgTypes = (`elem` [numericOid, int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder =
+  {-# INLINE fastFieldDecoder #-}
+  fastFieldDecoder =
     RowDecoder
       { fullRowDecoder = \case
           [singleColInfo] ->
             if singleColInfo.fieldTypeOid /= numericOid
               then
-                flip scientific 0 . fromIntegral <$> (inlinedSingleFieldRowDecoder @Int64).fullRowDecoder [singleColInfo]
+                fmap (flip scientific 0 . fromIntegral) <$> (fastFieldDecoder @Int64).fullRowDecoder [singleColInfo]
               else numericRowParser
           _ -> error "singleField expected a single column OID but got 0 or >1",
         rowColumnsTypeCheck = \case
@@ -1088,19 +1118,15 @@ boolRowDecoder =
 
 instance FromPgField Bool where
   fieldDecoder = parsePgType "Bool" [boolOid] $ \bs -> Right $ bs == binaryTrue
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = nonNullableRowDec "Bool" boolRowDecoder
+  {-# INLINE fastFieldDecoder #-}
+  fastFieldDecoder = boolRowDecoder
 
 -- instance {-# OVERLAPPING #-} FromPgField (Maybe Bool) where
 --   -- This overlapping instance isn't pretty, but it reduces memory
 --   -- usage and improves performance a bit
 --   fieldDecoder = error "TODO Maybe Bool"
---   {-# NOINLINE singleFieldRowDecoder #-}
---   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
---   {-# INLINE inlinedSingleFieldRowDecoder #-}
---   inlinedSingleFieldRowDecoder = boolRowDecoder
+--   {-# INLINE fastFieldDecoder #-}
+--   fastFieldDecoder = boolRowDecoder
 
 instance FromPgField Char where
   fieldDecoder =
@@ -1140,19 +1166,15 @@ textDecoder =
 instance FromPgField Text where
   -- TODO: Use some faster unsafeDecodeUtf8 function?
   fieldDecoder = parsePgType "Text" [textOid, varcharOid, nameOid] $ \bs -> Right $ decodeUtf8 bs
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = nonNullableRowDec "Text" textDecoder
+  {-# INLINE fastFieldDecoder #-}
+  fastFieldDecoder = textDecoder
 
 -- instance {-# OVERLAPPING #-} FromPgField (Maybe Text) where
 --   -- This overlapping instance isn't pretty, but it reduces memory
 --   -- usage and improves performance a bit
 --   fieldDecoder = error "TODO Maybe Text"
---   {-# NOINLINE singleFieldRowDecoder #-}
---   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
---   {-# INLINE inlinedSingleFieldRowDecoder #-}
---   inlinedSingleFieldRowDecoder = textDecoder
+--   {-# INLINE fastFieldDecoder #-}
+--   fastFieldDecoder = textDecoder
 
 instance FromPgField LT.Text where
   -- TODO: Use some faster unsafeDecodeUtf8 function?
@@ -1198,19 +1220,15 @@ instance FromPgField UTCTime where
       let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
           parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
       Right $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-  {-# NOINLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = nonNullableRowDec "UTCTime" utcTimeRowDecoder
+  {-# NOINLINE fastFieldDecoder #-}
+  fastFieldDecoder = utcTimeRowDecoder
 
 -- instance {-# OVERLAPPING #-} FromPgField (Maybe UTCTime) where
 --   -- This overlapping instance isn't pretty, but it reduces memory
 --   -- usage and improves performance a bit
 --   fieldDecoder = error "TODO Maybe UTCTime"
---   {-# NOINLINE singleFieldRowDecoder #-}
---   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
---   {-# INLINE inlinedSingleFieldRowDecoder #-}
---   inlinedSingleFieldRowDecoder = utcTimeRowDecoder
+--   {-# INLINE fastFieldDecoder #-}
+--   fastFieldDecoder = utcTimeRowDecoder
 
 instance FromPgField (Unbounded UTCTime) where
   fieldDecoder = parsePgType "Unbounded UTCTime" [timestamptzOid] $ \case
@@ -1281,8 +1299,8 @@ instance FromPgField Day where
       -- But I found a simpler way to do this. Let's see if it works in our property based tests
       jd <- BinSer.decodeInt32BE 0 bs
       Right $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
+  {-# INLINE fastFieldDecoder #-}
+  fastFieldDecoder = dayRowDecoder
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder = nonNullableRowDec "Day" dayRowDecoder
 
@@ -1290,10 +1308,8 @@ instance FromPgField Day where
 --   -- This overlapping instance isn't pretty, but it reduces memory
 --   -- usage and improves performance a bit
 --   fieldDecoder = error "TODO Maybe Day"
---   {-# NOINLINE singleFieldRowDecoder #-}
---   singleFieldRowDecoder = inlinedSingleFieldRowDecoder
---   {-# INLINE inlinedSingleFieldRowDecoder #-}
---   inlinedSingleFieldRowDecoder = dayRowDecoder
+--   {-# INLINE fastFieldDecoder #-}
+--   fastFieldDecoder = dayRowDecoder
 
 instance FromPgField (Unbounded Day) where
   fieldDecoder = parsePgType "Unbounded Day" [dateOid] $ \case
@@ -1366,37 +1382,41 @@ nonNullableRowDec haskellTypeName rdec =
 instance (FromPgField a) => FromPgField (Maybe a) where
   fieldDecoder = nullableField fieldDecoder
 
-  {-# NOINLINE singleFieldRowDecoder #-}
-  singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder =
-    RowDecoder
-      { fullRowDecoder = \finfos ->
-          let frd = inlinedSingleFieldRowDecoder.fullRowDecoder finfos
-           in do
-                -- TODO: We're decoding the field length twice with
-                -- the peek call when the value isn't NULL.
-                -- Maybe we should make `FromPgField`'s new methods
-                -- be two `Parser` objects: one for both length and field
-                -- and another only for the field (but how would that work
-                -- without the length..? It wouldn't.)
-                -- Maybe we do the `Parser (Maybe a)` for `a` types, then.
-                -- We can build a `Parser a` from that with `decodesSqlNullTo`
-                -- and with inlining there's nothing to lose?
-                fieldLen <- Parser.peekInt32BE
-                if fieldLen == (-1)
-                  then case fieldDecoder.decodesSqlNullTo of
-                    Left err -> fail err
-                    Right v -> Parser.skip 4 >> pure v
-                  else do
-                    Just <$> frd,
-        rowColumnsTypeCheck =
-          let fdec = fieldDecoder @a
-           in \case
-                [singleColInfo] -> [(singleColInfo, fdec.allowedPgTypes singleColInfo)]
-                _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-        numExpectedColumns = 1
-      }
+  {-# INLINE fastFieldDecoder #-}
+  fastFieldDecoder =
+    fastFieldDecoder <&> \case
+      Nothing -> Nothing
+      Just v -> Just (Just v)
+
+-- let ffdec = fastFieldDecoder @a
+-- in
+-- RowDecoder
+--   { fullRowDecoder = \finfos ->
+--       let frd = fastFieldDecoder.fullRowDecoder finfos
+--        in do
+--             -- TODO: We're decoding the field length twice with
+--             -- the peek call when the value isn't NULL.
+--             -- Maybe we should make `FromPgField`'s new methods
+--             -- be two `Parser` objects: one for both length and field
+--             -- and another only for the field (but how would that work
+--             -- without the length..? It wouldn't.)
+--             -- Maybe we do the `Parser (Maybe a)` for `a` types, then.
+--             -- We can build a `Parser a` from that with `decodesSqlNullTo`
+--             -- and with inlining there's nothing to lose?
+--             fieldLen <- Parser.peekInt32BE
+--             if fieldLen == (-1)
+--               then case fieldDecoder.decodesSqlNullTo of
+--                 Left err -> fail err
+--                 Right v -> Parser.skip 4 >> pure v
+--               else do
+--                 Just <$> frd,
+--     rowColumnsTypeCheck =
+--       let fdec = fieldDecoder @a
+--        in \case
+--             [singleColInfo] -> [(singleColInfo, fdec.allowedPgTypes singleColInfo)]
+--             _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+--     numExpectedColumns = 1
+--   }
 
 allowOnlyArrayTypes :: FieldInfo -> Bool
 allowOnlyArrayTypes fieldInfo =

From e42a7bc623cef05400c77e35e4ea4146879dc34c Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Wed, 19 Aug 2026 15:26:08 -0300
Subject: [PATCH 15/22] Some memory usage improvements

Let's see if this is better than the OVERLAPPING instances
---
 hpgsql/src/Hpgsql/Encoding.hs | 275 +++++++++++++++++++---------------
 1 file changed, 155 insertions(+), 120 deletions(-)

diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index 6592c42..c03253c 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -28,6 +28,7 @@ module Hpgsql.Encoding
     RowDecoder (..), -- TODO: Can we export ctor?
     singleField,
     singleFieldRowDecoder,
+    inlinedSingleFieldRowDecoder,
     nullableField,
     genericFromPgRow,
 
@@ -79,7 +80,6 @@ import Data.CaseInsensitive (CI)
 import qualified Data.CaseInsensitive as CI
 import Data.Coerce (coerce)
 import Data.Fixed (divMod')
-import Data.Functor ((<&>))
 import Data.Functor.Contravariant (Contravariant (..))
 import Data.Int (Int16, Int32, Int64)
 import qualified Data.List as List
@@ -176,6 +176,8 @@ singleField fdec =
                     case decode nextColBs of
                       Right v -> pure v
                       Left err -> fail err
+                  -- This `case` is why we require `fieldAndValueDecoder` to decode
+                  -- SQL NULL into `Nothing`: we do check decodesSqlNullTo.
                   else case fdec.decodesSqlNullTo of
                     Right v -> pure v
                     Left err -> fail err
@@ -222,48 +224,67 @@ class FromPgField a where
   -- Any implementation of this _must_ return a `Nothing` for a SQL NULL value,
   -- regardless of what `FieldDecoder` would do with a SQL NULL.
   -- TODO: Move this to inside the FieldDecoder type?
-  {-# INLINE fastFieldDecoder #-}
-  fastFieldDecoder :: RowDecoder (Maybe a)
-  fastFieldDecoder =
-    let fdec = fieldDecoder
-     in RowDecoder
-          { fullRowDecoder = \case
-              [singleColInfo] -> do
-                len <- Parser.takeInt32BE
-                if len == (-1)
-                  then pure Nothing
-                  else do
-                    bs <- Parser.take (fromIntegral len)
-                    case fdec.fieldValueDecoder singleColInfo bs of
-                      Left err -> fail err
-                      Right v -> pure v
-              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-            rowColumnsTypeCheck = \case
-              [singleColInfo] -> [(singleColInfo, fdec.allowedPgTypes singleColInfo)]
-              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-            numExpectedColumns = 1
-          }
+  {-# NOINLINE fieldAndValueDecoder #-}
+  fieldAndValueDecoder :: RowDecoder (Maybe a)
+  fieldAndValueDecoder =
+    RowDecoder
+      { fullRowDecoder =
+          case inlinedConstFieldDecoder of
+            Nothing -> slowerParser
+            Just fd -> const fd,
+        rowColumnsTypeCheck = \case
+          [singleColInfo] -> [(singleColInfo, (fieldDecoder @a).allowedPgTypes singleColInfo)]
+          _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+        numExpectedColumns = 1
+      }
+    where
+      -- slowerParser takes a ByteString and passes it to the
+      -- field decoder.
+      slowerParser = \case
+        [singleColInfo] -> do
+          len <- Parser.takeInt32BE
+          if len == (-1)
+            then pure Nothing
+            else do
+              bs <- Parser.take (fromIntegral len)
+              case fieldDecoder.fieldValueDecoder singleColInfo bs of
+                Left err -> fail err
+                Right v -> pure v
+        _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1"
+
+  {-# INLINE inlinedConstFieldDecoder #-}
+
+  -- | For types where there is a fast way to decode fields+values
+  -- without knowing the OID of the value in the query (of course, the
+  -- possible OIDs are still limited by the FieldDecoder's allowed types),
+  -- this can help provide a significant boost to inlined row decoders.
+  -- Define as `Nothing` if this isn't possible.
+  inlinedConstFieldDecoder :: Maybe (Parser.Parser (Maybe a))
+  inlinedConstFieldDecoder = Nothing
 
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder :: RowDecoder a
-  inlinedSingleFieldRowDecoder =
-    let fastrdec = fastFieldDecoder @a
-     in do
-          RowDecoder
-            { fullRowDecoder = \finfos -> do
-                mv <- fastrdec.fullRowDecoder finfos
+  inlinedSingleFieldRowDecoder = case inlinedConstFieldDecoder @a of
+    Nothing -> singleField fieldDecoder
+    Just p ->
+      let fdec = fieldDecoder @a
+       in RowDecoder
+            { fullRowDecoder = const $ do
+                mv <- p
                 case mv of
-                  -- This `case` is why we require `fastFieldDecoder` to decode
-                  -- SQL NULL into `Nothing`: we do check decodesSqlNullTo.
-                  Nothing -> case fieldDecoder.decodesSqlNullTo of
-                    Left err -> fail err -- Type doesn't accept NULLs
+                  Nothing -> case fdec.decodesSqlNullTo of
+                    Left err -> fail err
                     Right v -> pure v
                   Just v -> pure v,
-              rowColumnsTypeCheck = fastrdec.rowColumnsTypeCheck,
-              numExpectedColumns = fastrdec.numExpectedColumns
+              rowColumnsTypeCheck = \case
+                [singleColInfo] -> [(singleColInfo, fdec.allowedPgTypes singleColInfo)]
+                _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+              numExpectedColumns = 1
             }
 
--- TODO: better name for `singleFieldRowDecoder`?
+-- TODO: better name for `singleFieldRowDecoder`? We have 3 methods now
+-- to create a single field RowDecoder, what a mess! Figure out names
+-- and code docs.
 {-# NOINLINE singleFieldRowDecoder #-}
 singleFieldRowDecoder :: forall a. (FromPgField a) => RowDecoder a
 singleFieldRowDecoder = inlinedSingleFieldRowDecoder
@@ -848,20 +869,20 @@ instance FromPgField () where
 
 -- TODO: Inline intRowDecoder into FromPgField? And all others too?
 {-# INLINE intRowDecoder #-}
-intRowDecoder :: RowDecoder (Maybe Int)
-intRowDecoder =
-  inlinableRowDecoder haskellIntOids $ do
-    fieldLen <- Parser.takeInt32BE
-    -- TODO: We're assuming `Int` is always 64 bits, so 64bit CPUs? Is that ok?
-    -- TODO: Is there a way to optimistically assume <=4 bytes and use our custom new parser?
-    case fieldLen of
-      4 -> Just . fromIntegral <$> Parser.takeInt32BE
-      (-1) -> pure Nothing
-      8 -> Just . fromIntegral <$> Parser.takeInt64BE
-      2 -> Just . fromIntegral <$> Parser.takeInt16BE
-      _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
+intRowDecoder :: Parser.Parser (Maybe Int)
+intRowDecoder = do
+  fieldLen <- Parser.takeInt32BE
+  -- TODO: We're assuming `Int` is always 64 bits, so 64bit CPUs? Is that ok?
+  -- TODO: Is there a way to optimistically assume <=4 bytes and use our custom new parser?
+  case fieldLen of
+    4 -> Just . fromIntegral <$> Parser.takeInt32BE
+    (-1) -> pure Nothing
+    8 -> Just . fromIntegral <$> Parser.takeInt64BE
+    2 -> Just . fromIntegral <$> Parser.takeInt16BE
+    _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
 
 instance FromPgField Int where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
@@ -870,10 +891,11 @@ instance FromPgField Int where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Int type. Use a `Maybe Int`",
         allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
       }
-  {-# INLINE fastFieldDecoder #-}
-  fastFieldDecoder = intRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = nonNullableRowDec "Int" intRowDecoder
+
+  -- {-# INLINE fieldAndValueDecoder #-}
+  -- fieldAndValueDecoder = intRowDecoder
+  {-# INLINE inlinedConstFieldDecoder #-}
+  inlinedConstFieldDecoder = Just intRowDecoder
 
 -- instance {-# OVERLAPPING #-} FromPgField (Maybe Int) where
 --   -- This overlapping instance isn't pretty, but it reduces memory
@@ -888,8 +910,8 @@ instance FromPgField Int where
 --   --             Nothing -> Right Nothing,
 --   --     allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
 --   --   }
---   {-# INLINE fastFieldDecoder #-}
---   fastFieldDecoder = intRowDecoder
+--   {-# INLINE fieldAndValueDecoder #-}
+--   fieldAndValueDecoder = intRowDecoder
 
 instance FromPgField Int16 where
   fieldDecoder =
@@ -919,8 +941,8 @@ instance FromPgField Int32 where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Int32 type. Use a `Maybe Int32`",
         allowedPgTypes = (`elem` [int2Oid, int4Oid]) . fieldTypeOid
       }
-  {-# INLINE fastFieldDecoder #-}
-  fastFieldDecoder = int32RowDecoder
+  {-# INLINE fieldAndValueDecoder #-}
+  fieldAndValueDecoder = int32RowDecoder
 
 {-# INLINE int64RowDecoder #-}
 int64RowDecoder :: RowDecoder (Maybe Int64)
@@ -941,8 +963,8 @@ instance FromPgField Int64 where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Int64 type. Use a `Maybe Int64`",
         allowedPgTypes = (`elem` [int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
-  {-# INLINE fastFieldDecoder #-}
-  fastFieldDecoder = int64RowDecoder
+  {-# INLINE fieldAndValueDecoder #-}
+  fieldAndValueDecoder = int64RowDecoder
 
 instance FromPgField Integer where
   fieldDecoder =
@@ -970,33 +992,32 @@ instance FromPgField Oid where
         allowedPgTypes = (== oidOid) . fieldTypeOid
       }
 
-{-# INLINE floatRowDecoder #-}
-floatRowDecoder :: RowDecoder (Maybe Float)
-floatRowDecoder =
-  inlinableRowDecoder [float4Oid] Parser.takeFloatBEWithFieldLength
+-- {-# INLINE floatRowDecoder #-}
+-- floatRowDecoder :: Parser.Parser (Maybe Float)
+-- floatRowDecoder = Parser.takeFloatBEWithFieldLength
 
 instance FromPgField Float where
   fieldDecoder = parsePgType "Float" [float4Oid] $ Right . binaryFloat4Decoder
-  {-# INLINE fastFieldDecoder #-}
-  fastFieldDecoder = floatRowDecoder
+  -- {-# INLINE fieldAndValueDecoder #-}
+  -- fieldAndValueDecoder = floatRowDecoder
+  {-# INLINE inlinedConstFieldDecoder #-}
+  inlinedConstFieldDecoder = Just Parser.takeFloatBEWithFieldLength
 
 -- instance {-# OVERLAPPING #-} FromPgField (Maybe Float) where
 --   -- This overlapping instance isn't pretty, but it reduces memory
 --   -- usage and improves performance a bit
 --   fieldDecoder = error "TODO Maybe Float"
---   {-# INLINE fastFieldDecoder #-}
---   fastFieldDecoder = floatRowDecoder
+--   {-# INLINE fieldAndValueDecoder #-}
+--   fieldAndValueDecoder = floatRowDecoder
 
 {-# INLINE doubleRowDecoder #-}
-doubleRowDecoder :: RowDecoder (Maybe Double)
-doubleRowDecoder =
-  let float4OrDouble8Decoder = do
-        len <- Parser.takeInt32BE
-        case len of
-          8 -> Just <$> Parser.takeDoubleBE
-          4 -> Just . float2Double <$> Parser.takeFloatBE
-          _ -> pure Nothing
-   in inlinableRowDecoder [float8Oid, float4Oid] float4OrDouble8Decoder
+doubleRowDecoder :: Parser.Parser (Maybe Double)
+doubleRowDecoder = do
+  len <- Parser.takeInt32BE
+  case len of
+    8 -> Just <$> Parser.takeDoubleBE
+    4 -> Just . float2Double <$> Parser.takeFloatBE
+    _ -> pure Nothing
 
 instance FromPgField Double where
   fieldDecoder =
@@ -1009,17 +1030,18 @@ instance FromPgField Double where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Double type. Use a `Maybe Double`",
         allowedPgTypes = (`elem` [float8Oid, float4Oid]) . fieldTypeOid
       }
-  {-# INLINE fastFieldDecoder #-}
-  fastFieldDecoder = doubleRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = nonNullableRowDec "Double" doubleRowDecoder
+
+  -- {-# INLINE fieldAndValueDecoder #-}
+  -- fieldAndValueDecoder = doubleRowDecoder
+  {-# INLINE inlinedConstFieldDecoder #-}
+  inlinedConstFieldDecoder = Just doubleRowDecoder
 
 -- instance {-# OVERLAPPING #-} FromPgField (Maybe Double) where
 --   -- This overlapping instance isn't pretty, but it reduces memory
 --   -- usage and improves performance a bit
 --   fieldDecoder = error "TODO Maybe Double"
---   {-# INLINE fastFieldDecoder #-}
---   fastFieldDecoder = doubleRowDecoder
+--   {-# INLINE fieldAndValueDecoder #-}
+--   fieldAndValueDecoder = doubleRowDecoder
 
 -- | Allows you to specify a type (and other checks, possibly) for a `FieldDecoder`.
 -- This can be useful to ensure you're not accidentally decoding a different type.
@@ -1089,14 +1111,14 @@ instance FromPgField Scientific where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`",
         allowedPgTypes = (`elem` [numericOid, int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
-  {-# INLINE fastFieldDecoder #-}
-  fastFieldDecoder =
+  {-# INLINE fieldAndValueDecoder #-}
+  fieldAndValueDecoder =
     RowDecoder
       { fullRowDecoder = \case
           [singleColInfo] ->
             if singleColInfo.fieldTypeOid /= numericOid
               then
-                fmap (flip scientific 0 . fromIntegral) <$> (fastFieldDecoder @Int64).fullRowDecoder [singleColInfo]
+                fmap (flip scientific 0 . fromIntegral) <$> (fieldAndValueDecoder @Int64).fullRowDecoder [singleColInfo]
               else numericRowParser
           _ -> error "singleField expected a single column OID but got 0 or >1",
         rowColumnsTypeCheck = \case
@@ -1112,21 +1134,22 @@ binaryTrue :: ByteString
 binaryTrue = BinSer.encodePgBoolean True
 
 {-# INLINE boolRowDecoder #-}
-boolRowDecoder :: RowDecoder (Maybe Bool)
-boolRowDecoder =
-  inlinableRowDecoder [boolOid] $ fmap (== 1) <$> Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1
+boolRowDecoder :: Parser.Parser (Maybe Bool)
+boolRowDecoder = fmap (== 1) <$> Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1
 
 instance FromPgField Bool where
   fieldDecoder = parsePgType "Bool" [boolOid] $ \bs -> Right $ bs == binaryTrue
-  {-# INLINE fastFieldDecoder #-}
-  fastFieldDecoder = boolRowDecoder
+  -- {-# INLINE fieldAndValueDecoder #-}
+  -- fieldAndValueDecoder = boolRowDecoder
+  {-# INLINE inlinedConstFieldDecoder #-}
+  inlinedConstFieldDecoder = Just boolRowDecoder
 
 -- instance {-# OVERLAPPING #-} FromPgField (Maybe Bool) where
 --   -- This overlapping instance isn't pretty, but it reduces memory
 --   -- usage and improves performance a bit
 --   fieldDecoder = error "TODO Maybe Bool"
---   {-# INLINE fastFieldDecoder #-}
---   fastFieldDecoder = boolRowDecoder
+--   {-# INLINE fieldAndValueDecoder #-}
+--   fieldAndValueDecoder = boolRowDecoder
 
 instance FromPgField Char where
   fieldDecoder =
@@ -1154,9 +1177,8 @@ instance FromPgField LBS.ByteString where
   fieldDecoder = parsePgType "ByteString" [byteaOid] $ Right . LBS.fromStrict
 
 {-# INLINE textDecoder #-}
-textDecoder :: RowDecoder (Maybe Text)
-textDecoder =
-  inlinableRowDecoder [textOid, varcharOid, nameOid] $ do
+textDecoder :: Parser.Parser (Maybe Text)
+textDecoder = do
     len <- Parser.takeInt32BE
     if len >= 0
       -- TODO: Use some faster unsafeDecodeUtf8 function?
@@ -1166,15 +1188,17 @@ textDecoder =
 instance FromPgField Text where
   -- TODO: Use some faster unsafeDecodeUtf8 function?
   fieldDecoder = parsePgType "Text" [textOid, varcharOid, nameOid] $ \bs -> Right $ decodeUtf8 bs
-  {-# INLINE fastFieldDecoder #-}
-  fastFieldDecoder = textDecoder
+  -- {-# INLINE fieldAndValueDecoder #-}
+  -- fieldAndValueDecoder = textDecoder
+  {-# INLINE inlinedConstFieldDecoder #-}
+  inlinedConstFieldDecoder = Just textDecoder
 
 -- instance {-# OVERLAPPING #-} FromPgField (Maybe Text) where
 --   -- This overlapping instance isn't pretty, but it reduces memory
 --   -- usage and improves performance a bit
 --   fieldDecoder = error "TODO Maybe Text"
---   {-# INLINE fastFieldDecoder #-}
---   fastFieldDecoder = textDecoder
+--   {-# INLINE fieldAndValueDecoder #-}
+--   fieldAndValueDecoder = textDecoder
 
 instance FromPgField LT.Text where
   -- TODO: Use some faster unsafeDecodeUtf8 function?
@@ -1200,9 +1224,8 @@ instance FromPgField (CI String) where
   fieldDecoder = typeFieldDecoder (typeMustBeNamed "citext") $ CI.mk <$> fieldDecoder
 
 {-# INLINE utcTimeRowDecoder #-}
-utcTimeRowDecoder :: RowDecoder (Maybe UTCTime)
-utcTimeRowDecoder =
-  inlinableRowDecoder [timestamptzOid] $ do
+utcTimeRowDecoder :: Parser.Parser (Maybe UTCTime)
+utcTimeRowDecoder = do
     len <- Parser.takeInt32BE
     case len of
       8 -> do
@@ -1220,15 +1243,17 @@ instance FromPgField UTCTime where
       let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
           parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
       Right $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-  {-# NOINLINE fastFieldDecoder #-}
-  fastFieldDecoder = utcTimeRowDecoder
+  -- {-# NOINLINE fieldAndValueDecoder #-}
+  -- fieldAndValueDecoder = utcTimeRowDecoder
+  {-# INLINE inlinedConstFieldDecoder #-}
+  inlinedConstFieldDecoder = Just utcTimeRowDecoder
 
 -- instance {-# OVERLAPPING #-} FromPgField (Maybe UTCTime) where
 --   -- This overlapping instance isn't pretty, but it reduces memory
 --   -- usage and improves performance a bit
 --   fieldDecoder = error "TODO Maybe UTCTime"
---   {-# INLINE fastFieldDecoder #-}
---   fastFieldDecoder = utcTimeRowDecoder
+--   {-# INLINE fieldAndValueDecoder #-}
+--   fieldAndValueDecoder = utcTimeRowDecoder
 
 instance FromPgField (Unbounded UTCTime) where
   fieldDecoder = parsePgType "Unbounded UTCTime" [timestamptzOid] $ \case
@@ -1286,10 +1311,10 @@ instance FromPgField TimeOfDay where
       Right $ timeToTimeOfDay $ picosecondsToDiffTime $ fromIntegral usecs * 1_000_000
 
 {-# INLINE dayRowDecoder #-}
-dayRowDecoder :: RowDecoder (Maybe Day)
+dayRowDecoder :: Parser.Parser (Maybe Day)
 dayRowDecoder =
   let int32ToDay (i32 :: Int32) = let jd = fromIntegral i32 :: Integer in addJulianDurationClip (CalendarDiffDays 0 (jd - 13)) $ fromJulian 2000 01 01
-   in inlinableRowDecoder [dateOid] $ fmap int32ToDay <$> Parser.takeInt32BEWithFieldLength
+   in fmap int32ToDay <$> Parser.takeInt32BEWithFieldLength
 
 instance FromPgField Day where
   fieldDecoder = parsePgType "Day" [dateOid] $ \case
@@ -1299,17 +1324,18 @@ instance FromPgField Day where
       -- But I found a simpler way to do this. Let's see if it works in our property based tests
       jd <- BinSer.decodeInt32BE 0 bs
       Right $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
-  {-# INLINE fastFieldDecoder #-}
-  fastFieldDecoder = dayRowDecoder
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder = nonNullableRowDec "Day" dayRowDecoder
+
+  -- {-# INLINE fieldAndValueDecoder #-}
+  -- fieldAndValueDecoder = dayRowDecoder
+  {-# INLINE inlinedConstFieldDecoder #-}
+  inlinedConstFieldDecoder = Just dayRowDecoder
 
 -- instance {-# OVERLAPPING #-} FromPgField (Maybe Day) where
 --   -- This overlapping instance isn't pretty, but it reduces memory
 --   -- usage and improves performance a bit
 --   fieldDecoder = error "TODO Maybe Day"
---   {-# INLINE fastFieldDecoder #-}
---   fastFieldDecoder = dayRowDecoder
+--   {-# INLINE fieldAndValueDecoder #-}
+--   fieldAndValueDecoder = dayRowDecoder
 
 instance FromPgField (Unbounded Day) where
   fieldDecoder = parsePgType "Unbounded Day" [dateOid] $ \case
@@ -1382,17 +1408,26 @@ nonNullableRowDec haskellTypeName rdec =
 instance (FromPgField a) => FromPgField (Maybe a) where
   fieldDecoder = nullableField fieldDecoder
 
-  {-# INLINE fastFieldDecoder #-}
-  fastFieldDecoder =
-    fastFieldDecoder <&> \case
-      Nothing -> Nothing
-      Just v -> Just (Just v)
-
--- let ffdec = fastFieldDecoder @a
+  {-# INLINE inlinedConstFieldDecoder #-}
+  -- \| For types where there is a fast way to decode fields+values
+  -- without knowing the OID of the value in the query (of course, the
+  -- possible OIDs are still limited by the FieldDecoder's allowed types),
+  -- this can help provide a significant boost to inlined row decoders.
+  -- Define as `Nothing` if this isn't possible.
+  -- inlinedConstFieldDecoder :: Maybe (Parser.Parser (Maybe (Maybe a)))
+  inlinedConstFieldDecoder = case inlinedConstFieldDecoder @a of
+    Nothing -> Nothing
+    Just p -> Just $ do
+      mv <- p
+      case mv of
+        Nothing -> pure Nothing -- Must return Nothing for SQL Nulls
+        jv -> pure $ Just jv
+
+-- let ffdec = fieldAndValueDecoder @a
 -- in
 -- RowDecoder
 --   { fullRowDecoder = \finfos ->
---       let frd = fastFieldDecoder.fullRowDecoder finfos
+--       let frd = fieldAndValueDecoder.fullRowDecoder finfos
 --        in do
 --             -- TODO: We're decoding the field length twice with
 --             -- the peek call when the value isn't NULL.

From 2b91148774dbe4c05ea9d59ca69573e863edeefb Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Wed, 19 Aug 2026 16:27:43 -0300
Subject: [PATCH 16/22] Float decodesSqlNullTo outside and add strictness for
 better inlining

This is the prize I was looking for.

Now the row decoders are built with the NULL handling parts a lot more
inlined, which even means in a fully inlined row decoder we no longer
box into a `Maybe a` to then case match on it and fail on `Nothing`,
when the target record has a field typed as `a` (not a Maybe).
---
 hpgsql/src/Hpgsql/Encoding.hs | 76 +++++++++++++++++++++++++----------
 1 file changed, 55 insertions(+), 21 deletions(-)

diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index c03253c..eedbaa4 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -267,17 +267,18 @@ class FromPgField a where
   inlinedSingleFieldRowDecoder = case inlinedConstFieldDecoder @a of
     Nothing -> singleField fieldDecoder
     Just p ->
-      let fdec = fieldDecoder @a
+      let !valueForNull = case (fieldDecoder @a).decodesSqlNullTo of
+            Left err -> fail err
+            Right v -> pure v
+          !typeCheck = (fieldDecoder @a).allowedPgTypes
        in RowDecoder
             { fullRowDecoder = const $ do
                 mv <- p
                 case mv of
-                  Nothing -> case fdec.decodesSqlNullTo of
-                    Left err -> fail err
-                    Right v -> pure v
+                  Nothing -> valueForNull
                   Just v -> pure v,
               rowColumnsTypeCheck = \case
-                [singleColInfo] -> [(singleColInfo, fdec.allowedPgTypes singleColInfo)]
+                [singleColInfo] -> [(singleColInfo, typeCheck singleColInfo)]
                 _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
               numExpectedColumns = 1
             }
@@ -858,6 +859,7 @@ parsePgType !typeName !requiredTypeOids !fieldValueDecoder =
     }
 
 instance FromPgField () where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \_oid -> \case
@@ -914,6 +916,7 @@ instance FromPgField Int where
 --   fieldAndValueDecoder = intRowDecoder
 
 instance FromPgField Int16 where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder =
@@ -935,6 +938,7 @@ int32RowDecoder =
       _ -> fail "Trying to decode PG int4 but it's not 2 or 4 bytes long"
 
 instance FromPgField Int32 where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} -> binaryIntDecoder oid,
@@ -957,6 +961,7 @@ int64RowDecoder =
       _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
 
 instance FromPgField Int64 where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} -> binaryIntDecoder oid,
@@ -967,6 +972,7 @@ instance FromPgField Int64 where
   fieldAndValueDecoder = int64RowDecoder
 
 instance FromPgField Integer where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
@@ -983,6 +989,7 @@ instance FromPgField Integer where
       }
 
 instance FromPgField Oid where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \_ -> \case
@@ -997,7 +1004,9 @@ instance FromPgField Oid where
 -- floatRowDecoder = Parser.takeFloatBEWithFieldLength
 
 instance FromPgField Float where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Float" [float4Oid] $ Right . binaryFloat4Decoder
+
   -- {-# INLINE fieldAndValueDecoder #-}
   -- fieldAndValueDecoder = floatRowDecoder
   {-# INLINE inlinedConstFieldDecoder #-}
@@ -1020,6 +1029,7 @@ doubleRowDecoder = do
     _ -> pure Nothing
 
 instance FromPgField Double where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
@@ -1094,6 +1104,7 @@ numericRowParser = do
 
 instance FromPgField Scientific where
   -- See https://github.com/postgres/postgres/blob/799959dc7cf0e2462601bea8d07b6edec3fa0c4f/src/backend/utils/adt/numeric.c#L1163
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder = \FieldInfo {fieldTypeOid} ->
@@ -1128,6 +1139,7 @@ instance FromPgField Scientific where
       }
 
 instance FromPgField (Ratio Integer) where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = toRational <$> fieldDecoder @Scientific
 
 binaryTrue :: ByteString
@@ -1138,7 +1150,9 @@ boolRowDecoder :: Parser.Parser (Maybe Bool)
 boolRowDecoder = fmap (== 1) <$> Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1
 
 instance FromPgField Bool where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Bool" [boolOid] $ \bs -> Right $ bs == binaryTrue
+
   -- {-# INLINE fieldAndValueDecoder #-}
   -- fieldAndValueDecoder = boolRowDecoder
   {-# INLINE inlinedConstFieldDecoder #-}
@@ -1152,6 +1166,7 @@ instance FromPgField Bool where
 --   fieldAndValueDecoder = boolRowDecoder
 
 instance FromPgField Char where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     let textParser = fieldValueDecoder (fieldDecoder @Text)
      in FieldDecoder
@@ -1171,23 +1186,26 @@ instance FromPgField Char where
           }
 
 instance FromPgField ByteString where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "byteString" [byteaOid] Right
 
 instance FromPgField LBS.ByteString where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "ByteString" [byteaOid] $ Right . LBS.fromStrict
 
 {-# INLINE textDecoder #-}
 textDecoder :: Parser.Parser (Maybe Text)
 textDecoder = do
-    len <- Parser.takeInt32BE
-    if len >= 0
-      -- TODO: Use some faster unsafeDecodeUtf8 function?
-      then Just . decodeUtf8 <$> Parser.take (fromIntegral len)
-      else pure Nothing
+  len <- Parser.takeInt32BE
+  if len >= 0
+    -- TODO: Use some faster unsafeDecodeUtf8 function?
+    then Just . decodeUtf8 <$> Parser.take (fromIntegral len)
+    else pure Nothing
 
 instance FromPgField Text where
-  -- TODO: Use some faster unsafeDecodeUtf8 function?
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Text" [textOid, varcharOid, nameOid] $ \bs -> Right $ decodeUtf8 bs
+
   -- {-# INLINE fieldAndValueDecoder #-}
   -- fieldAndValueDecoder = textDecoder
   {-# INLINE inlinedConstFieldDecoder #-}
@@ -1201,41 +1219,45 @@ instance FromPgField Text where
 --   fieldAndValueDecoder = textDecoder
 
 instance FromPgField LT.Text where
-  -- TODO: Use some faster unsafeDecodeUtf8 function?
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Text" [textOid, varcharOid, nameOid] $ \bs -> Right $ LT.fromStrict $ decodeUtf8 bs
 
 instance FromPgField String where
-  -- TODO: Use some faster unsafeDecodeUtf8 function?
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "String" [textOid, varcharOid, nameOid] $ \bs -> Right $ Text.unpack $ decodeUtf8 bs
 
 -- | This instance does not work if you have fillTypeInfoCache disabled (that would be a non-default
 -- connection option).
 instance FromPgField (CI Text) where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = typeFieldDecoder (typeMustBeNamed "citext") $ CI.mk <$> fieldDecoder
 
 -- | This instance does not work if you have fillTypeInfoCache disabled (that would be a non-default
 -- connection option).
 instance FromPgField (CI LT.Text) where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = typeFieldDecoder (typeMustBeNamed "citext") $ CI.mk <$> fieldDecoder
 
 -- | This instance does not work if you have fillTypeInfoCache disabled (that would be a non-default
 -- connection option).
 instance FromPgField (CI String) where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = typeFieldDecoder (typeMustBeNamed "citext") $ CI.mk <$> fieldDecoder
 
 {-# INLINE utcTimeRowDecoder #-}
 utcTimeRowDecoder :: Parser.Parser (Maybe UTCTime)
 utcTimeRowDecoder = do
-    len <- Parser.takeInt32BE
-    case len of
-      8 -> do
-        totalusecs <- Parser.takeInt64BE
-        let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
-            parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
-        pure $ Just $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
-      _ -> pure Nothing
+  len <- Parser.takeInt32BE
+  case len of
+    8 -> do
+      totalusecs <- Parser.takeInt64BE
+      let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
+          parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
+      pure $ Just $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
+    _ -> pure Nothing
 
 instance FromPgField UTCTime where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "UTCTime" [timestamptzOid] $ \case
     bs -> do
       -- See https://github.com/postgres/postgres/blob/50cb7505b3010736b9a7922e903931534785f3aa/src/backend/utils/adt/timestamp.c#L1909
@@ -1243,6 +1265,7 @@ instance FromPgField UTCTime where
       let (day, timeusecs) = totalusecs `divMod` 86_400_000_000 -- USECS per day
           parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
       Right $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
+
   -- {-# NOINLINE fieldAndValueDecoder #-}
   -- fieldAndValueDecoder = utcTimeRowDecoder
   {-# INLINE inlinedConstFieldDecoder #-}
@@ -1256,6 +1279,7 @@ instance FromPgField UTCTime where
 --   fieldAndValueDecoder = utcTimeRowDecoder
 
 instance FromPgField (Unbounded UTCTime) where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Unbounded UTCTime" [timestamptzOid] $ \case
     bs -> do
       -- See https://github.com/postgres/postgres/blob/50cb7505b3010736b9a7922e903931534785f3aa/src/backend/utils/adt/timestamp.c#L1909
@@ -1272,6 +1296,7 @@ instance FromPgField (Unbounded UTCTime) where
                  in Finite $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
 
 instance FromPgField ZonedTime where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "ZonedTime" [timestamptzOid] $ \case
     bs -> do
       -- See https://github.com/postgres/postgres/blob/50cb7505b3010736b9a7922e903931534785f3aa/src/backend/utils/adt/timestamp.c#L1909
@@ -1281,6 +1306,7 @@ instance FromPgField ZonedTime where
       Right $ utcToZonedTime utc $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
 
 instance FromPgField (Unbounded ZonedTime) where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Unbounded ZonedTime" [timestamptzOid] $ \case
     bs -> do
       -- See https://github.com/postgres/postgres/blob/50cb7505b3010736b9a7922e903931534785f3aa/src/backend/utils/adt/timestamp.c#L1909
@@ -1297,6 +1323,7 @@ instance FromPgField (Unbounded ZonedTime) where
                  in Finite $ utcToZonedTime utc $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
 
 instance FromPgField LocalTime where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "LocalTime" [timestampOid] $ \case
     bs -> do
       totalusecs <- BinSer.decodeInt64BE 0 bs
@@ -1305,6 +1332,7 @@ instance FromPgField LocalTime where
       Right $ LocalTime parsedDate (timeToTimeOfDay $ picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
 
 instance FromPgField TimeOfDay where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "TimeOfDay" [timeOid] $ \case
     bs -> do
       usecs <- BinSer.decodeInt64BE 0 bs
@@ -1317,6 +1345,7 @@ dayRowDecoder =
    in fmap int32ToDay <$> Parser.takeInt32BEWithFieldLength
 
 instance FromPgField Day where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Day" [dateOid] $ \case
     bs -> do
       -- There is a very specific conversion function for these, which I poorly translated to Haskell
@@ -1338,6 +1367,7 @@ instance FromPgField Day where
 --   fieldAndValueDecoder = dayRowDecoder
 
 instance FromPgField (Unbounded Day) where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Unbounded Day" [dateOid] $ \case
     bs -> do
       -- There is a very specific conversion function for these, which I poorly translated to Haskell
@@ -1354,6 +1384,7 @@ instance FromPgField (Unbounded Day) where
                 Finite $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
 
 instance FromPgField CalendarDiffTime where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "CalendarDiffTime " [intervalOid] $ \bs -> do
     nMicrosecs <- BinSer.decodeInt64BE 0 bs
     nDays <- BinSer.decodeInt32BE 8 bs
@@ -1361,12 +1392,14 @@ instance FromPgField CalendarDiffTime where
     Right $ CalendarDiffTime {ctMonths = fromIntegral nMonths, ctTime = secondsToNominalDiffTime (fromIntegral nDays * 86400) + realToFrac (picosecondsToDiffTime (fromIntegral nMicrosecs * 1_000_000))}
 
 instance FromPgField UUID where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "UUID" [uuidOid] $ \case
     bs -> case UUID.fromByteString (LBS.fromStrict bs) of
       Just uuid -> Right uuid
       Nothing -> Left "Bug in Hpgsql: UUID field could not be decoded"
 
 instance FromPgField Aeson.Value where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder =
@@ -1406,6 +1439,7 @@ nonNullableRowDec haskellTypeName rdec =
         }
 
 instance (FromPgField a) => FromPgField (Maybe a) where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = nullableField fieldDecoder
 
   {-# INLINE inlinedConstFieldDecoder #-}

From cc7592104043b3f9dc25b1dbc50a47dafb77a104 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Wed, 19 Aug 2026 16:54:59 -0300
Subject: [PATCH 17/22] Tidy up, a few more INLINE pragmas

---
 TODO.md                       |   5 +-
 hpgsql/src/Hpgsql/Encoding.hs | 182 ++++++----------------------------
 hpgsql/src/Hpgsql/Types.hs    |  23 +++--
 3 files changed, 49 insertions(+), 161 deletions(-)

diff --git a/TODO.md b/TODO.md
index e3e3aa8..6d50f38 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,5 +1,2 @@
 - Test both `singleField fieldDecoder` and `singleFieldRowDecoder` for every type in our tests.
-- Make every FromPgField instance have a dedicated singleFieldRowDecoder override, change our benchmarks to exercise other types we're not, like `numeric` and `Float`
-- Investigate why overlapping (Maybe a) instance is better for record decoding but worse for Tuple decoding
-  - Revert things: derive the overlapping (Maybe a) instance, derive the `FromPgField a` using that under the hood.
-- Try to achieve a 100% inlined row decoder for a small record type
+- Some types (the Aeson ones, for example) still don't derive specialized row decoders
diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index eedbaa4..60cd2e8 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -27,8 +27,6 @@ module Hpgsql.Encoding
     FromPgRow (..),
     RowDecoder (..), -- TODO: Can we export ctor?
     singleField,
-    singleFieldRowDecoder,
-    inlinedSingleFieldRowDecoder,
     nullableField,
     genericFromPgRow,
 
@@ -252,21 +250,30 @@ class FromPgField a where
                 Right v -> pure v
         _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1"
 
-  {-# INLINE inlinedConstFieldDecoder #-}
-
   -- | For types where there is a fast way to decode fields+values
   -- without knowing the OID of the value in the query (of course, the
   -- possible OIDs are still limited by the FieldDecoder's allowed types),
   -- this can help provide a significant boost to inlined row decoders.
   -- Define as `Nothing` if this isn't possible.
+  {-# INLINE inlinedConstFieldDecoder #-}
   inlinedConstFieldDecoder :: Maybe (Parser.Parser (Maybe a))
   inlinedConstFieldDecoder = Nothing
 
+  -- | Semantically equivalent to `singleField fieldDecoder`, but for
+  -- some types it can provide a much faster `RowDecoder`. Beware that
+  -- this will produce more code in row decoders.
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder :: RowDecoder a
   inlinedSingleFieldRowDecoder = case inlinedConstFieldDecoder @a of
+    -- This is a class method instead of a top-level function
+    -- because the GHC inliner behaves differently when it's a top-level
+    -- function, and benchmarks show it gets worse.
     Nothing -> singleField fieldDecoder
     Just p ->
+      -- The strictness and floating out of fieldDecoder-derived
+      -- values allows GHC to inline a lot more. For example, `valueForNull`
+      -- gets inlined to a `fail "Cannot decode SQL NULL ..."` for basic types
+      -- like `Int`.
       let !valueForNull = case (fieldDecoder @a).decodesSqlNullTo of
             Left err -> fail err
             Right v -> pure v
@@ -283,13 +290,6 @@ class FromPgField a where
               numExpectedColumns = 1
             }
 
--- TODO: better name for `singleFieldRowDecoder`? We have 3 methods now
--- to create a single field RowDecoder, what a mess! Figure out names
--- and code docs.
-{-# NOINLINE singleFieldRowDecoder #-}
-singleFieldRowDecoder :: forall a. (FromPgField a) => RowDecoder a
-singleFieldRowDecoder = inlinedSingleFieldRowDecoder
-
 class FromPgRow a where
   rowDecoder :: RowDecoder a
   default rowDecoder :: (Generic a, ProductTypeDecoder (Rep a)) => RowDecoder a
@@ -363,43 +363,43 @@ compositeTypeEncoder rowEnc =
     }
 
 instance (FromPgField a) => FromPgRow (Only a) where
-  rowDecoder = Only <$> singleFieldRowDecoder
+  rowDecoder = Only <$> inlinedSingleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b) => FromPgRow (a, b) where
-  rowDecoder = (,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder
+  rowDecoder = (,) <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c) => FromPgRow (a, b, c) where
-  rowDecoder = (,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+  rowDecoder = (,,) <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d) => FromPgRow (a, b, c, d) where
-  rowDecoder = (,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+  rowDecoder = (,,,) <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e) => FromPgRow (a, b, c, d, e) where
-  rowDecoder = (,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+  rowDecoder = (,,,,) <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f) => FromPgRow (a, b, c, d, e, f) where
-  rowDecoder = (,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+  rowDecoder = (,,,,,) <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g) => FromPgRow (a, b, c, d, e, f, g) where
-  rowDecoder = (,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+  rowDecoder = (,,,,,,) <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g, FromPgField h) => FromPgRow (a, b, c, d, e, f, g, h) where
-  rowDecoder = (,,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+  rowDecoder = (,,,,,,,) <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g, FromPgField h, FromPgField i) => FromPgRow (a, b, c, d, e, f, g, h, i) where
-  rowDecoder = (,,,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+  rowDecoder = (,,,,,,,,) <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g, FromPgField h, FromPgField i, FromPgField j) => FromPgRow (a, b, c, d, e, f, g, h, i, j) where
-  rowDecoder = (,,,,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+  rowDecoder = (,,,,,,,,,) <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g, FromPgField h, FromPgField i, FromPgField j, FromPgField k) => FromPgRow (a, b, c, d, e, f, g, h, i, j, k) where
-  rowDecoder = (,,,,,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+  rowDecoder = (,,,,,,,,,,) <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g, FromPgField h, FromPgField i, FromPgField j, FromPgField k, FromPgField l) => FromPgRow (a, b, c, d, e, f, g, h, i, j, k, l) where
-  rowDecoder = (,,,,,,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+  rowDecoder = (,,,,,,,,,,,) <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 instance (FromPgField a, FromPgField b, FromPgField c, FromPgField d, FromPgField e, FromPgField f, FromPgField g, FromPgField h, FromPgField i, FromPgField j, FromPgField k, FromPgField l, FromPgField m) => FromPgRow (a, b, c, d, e, f, g, h, i, j, k, l, m) where
-  rowDecoder = (,,,,,,,,,,,,) <$> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder <*> singleFieldRowDecoder
+  rowDecoder = (,,,,,,,,,,,,) <$> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder <*> inlinedSingleFieldRowDecoder
 
 data FieldEncoder a = FieldEncoder
   { toTypeOid :: !(EncodingContext -> Maybe Oid),
@@ -894,27 +894,9 @@ instance FromPgField Int where
         allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
       }
 
-  -- {-# INLINE fieldAndValueDecoder #-}
-  -- fieldAndValueDecoder = intRowDecoder
   {-# INLINE inlinedConstFieldDecoder #-}
   inlinedConstFieldDecoder = Just intRowDecoder
 
--- instance {-# OVERLAPPING #-} FromPgField (Maybe Int) where
---   -- This overlapping instance isn't pretty, but it reduces memory
---   -- usage and improves performance a bit
---   fieldDecoder = error "TODO Maybe Int"
-
---   -- FieldDecoder
---   --   { fieldValueDecoder = \FieldInfo {fieldTypeOid = oid} ->
---   --       let !decode = binaryIntDecoder oid
---   --        in \case
---   --             Just bs -> Just <$> decode bs
---   --             Nothing -> Right Nothing,
---   --     allowedPgTypes = (`elem` haskellIntOids) . fieldTypeOid
---   --   }
---   {-# INLINE fieldAndValueDecoder #-}
---   fieldAndValueDecoder = intRowDecoder
-
 instance FromPgField Int16 where
   {-# INLINE fieldDecoder #-}
   fieldDecoder =
@@ -999,26 +981,13 @@ instance FromPgField Oid where
         allowedPgTypes = (== oidOid) . fieldTypeOid
       }
 
--- {-# INLINE floatRowDecoder #-}
--- floatRowDecoder :: Parser.Parser (Maybe Float)
--- floatRowDecoder = Parser.takeFloatBEWithFieldLength
-
 instance FromPgField Float where
   {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Float" [float4Oid] $ Right . binaryFloat4Decoder
 
-  -- {-# INLINE fieldAndValueDecoder #-}
-  -- fieldAndValueDecoder = floatRowDecoder
   {-# INLINE inlinedConstFieldDecoder #-}
   inlinedConstFieldDecoder = Just Parser.takeFloatBEWithFieldLength
 
--- instance {-# OVERLAPPING #-} FromPgField (Maybe Float) where
---   -- This overlapping instance isn't pretty, but it reduces memory
---   -- usage and improves performance a bit
---   fieldDecoder = error "TODO Maybe Float"
---   {-# INLINE fieldAndValueDecoder #-}
---   fieldAndValueDecoder = floatRowDecoder
-
 {-# INLINE doubleRowDecoder #-}
 doubleRowDecoder :: Parser.Parser (Maybe Double)
 doubleRowDecoder = do
@@ -1041,18 +1010,9 @@ instance FromPgField Double where
         allowedPgTypes = (`elem` [float8Oid, float4Oid]) . fieldTypeOid
       }
 
-  -- {-# INLINE fieldAndValueDecoder #-}
-  -- fieldAndValueDecoder = doubleRowDecoder
   {-# INLINE inlinedConstFieldDecoder #-}
   inlinedConstFieldDecoder = Just doubleRowDecoder
 
--- instance {-# OVERLAPPING #-} FromPgField (Maybe Double) where
---   -- This overlapping instance isn't pretty, but it reduces memory
---   -- usage and improves performance a bit
---   fieldDecoder = error "TODO Maybe Double"
---   {-# INLINE fieldAndValueDecoder #-}
---   fieldAndValueDecoder = doubleRowDecoder
-
 -- | Allows you to specify a type (and other checks, possibly) for a `FieldDecoder`.
 -- This can be useful to ensure you're not accidentally decoding a different type.
 --
@@ -1153,18 +1113,9 @@ instance FromPgField Bool where
   {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Bool" [boolOid] $ \bs -> Right $ bs == binaryTrue
 
-  -- {-# INLINE fieldAndValueDecoder #-}
-  -- fieldAndValueDecoder = boolRowDecoder
   {-# INLINE inlinedConstFieldDecoder #-}
   inlinedConstFieldDecoder = Just boolRowDecoder
 
--- instance {-# OVERLAPPING #-} FromPgField (Maybe Bool) where
---   -- This overlapping instance isn't pretty, but it reduces memory
---   -- usage and improves performance a bit
---   fieldDecoder = error "TODO Maybe Bool"
---   {-# INLINE fieldAndValueDecoder #-}
---   fieldAndValueDecoder = boolRowDecoder
-
 instance FromPgField Char where
   {-# INLINE fieldDecoder #-}
   fieldDecoder =
@@ -1206,18 +1157,9 @@ instance FromPgField Text where
   {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Text" [textOid, varcharOid, nameOid] $ \bs -> Right $ decodeUtf8 bs
 
-  -- {-# INLINE fieldAndValueDecoder #-}
-  -- fieldAndValueDecoder = textDecoder
   {-# INLINE inlinedConstFieldDecoder #-}
   inlinedConstFieldDecoder = Just textDecoder
 
--- instance {-# OVERLAPPING #-} FromPgField (Maybe Text) where
---   -- This overlapping instance isn't pretty, but it reduces memory
---   -- usage and improves performance a bit
---   fieldDecoder = error "TODO Maybe Text"
---   {-# INLINE fieldAndValueDecoder #-}
---   fieldAndValueDecoder = textDecoder
-
 instance FromPgField LT.Text where
   {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Text" [textOid, varcharOid, nameOid] $ \bs -> Right $ LT.fromStrict $ decodeUtf8 bs
@@ -1266,18 +1208,9 @@ instance FromPgField UTCTime where
           parsedDate = addJulianDurationClip (CalendarDiffDays 0 (fromIntegral day)) $ fromJulian 1999 12 19
       Right $ UTCTime parsedDate (picosecondsToDiffTime $ fromIntegral timeusecs * 1_000_000)
 
-  -- {-# NOINLINE fieldAndValueDecoder #-}
-  -- fieldAndValueDecoder = utcTimeRowDecoder
   {-# INLINE inlinedConstFieldDecoder #-}
   inlinedConstFieldDecoder = Just utcTimeRowDecoder
 
--- instance {-# OVERLAPPING #-} FromPgField (Maybe UTCTime) where
---   -- This overlapping instance isn't pretty, but it reduces memory
---   -- usage and improves performance a bit
---   fieldDecoder = error "TODO Maybe UTCTime"
---   {-# INLINE fieldAndValueDecoder #-}
---   fieldAndValueDecoder = utcTimeRowDecoder
-
 instance FromPgField (Unbounded UTCTime) where
   {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Unbounded UTCTime" [timestamptzOid] $ \case
@@ -1354,18 +1287,9 @@ instance FromPgField Day where
       jd <- BinSer.decodeInt32BE 0 bs
       Right $ addJulianDurationClip (CalendarDiffDays 0 (fromIntegral jd - 13)) $ fromJulian 2000 01 01
 
-  -- {-# INLINE fieldAndValueDecoder #-}
-  -- fieldAndValueDecoder = dayRowDecoder
   {-# INLINE inlinedConstFieldDecoder #-}
   inlinedConstFieldDecoder = Just dayRowDecoder
 
--- instance {-# OVERLAPPING #-} FromPgField (Maybe Day) where
---   -- This overlapping instance isn't pretty, but it reduces memory
---   -- usage and improves performance a bit
---   fieldDecoder = error "TODO Maybe Day"
---   {-# INLINE fieldAndValueDecoder #-}
---   fieldAndValueDecoder = dayRowDecoder
-
 instance FromPgField (Unbounded Day) where
   {-# INLINE fieldDecoder #-}
   fieldDecoder = parsePgType "Unbounded Day" [dateOid] $ \case
@@ -1404,12 +1328,14 @@ instance FromPgField Aeson.Value where
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let -- jsonb has a byte prepended to the contents and json does not
-                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in \case
-                  bs -> case Aeson.decodeStrict $ fixJsonb bs of
-                    Just d -> Right d
-                    Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid.",
+            let
+              -- jsonb has a byte prepended to the contents and json does not
+              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in
+              \case
+                bs -> case Aeson.decodeStrict $ fixJsonb bs of
+                  Just d -> Right d
+                  Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid.",
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
@@ -1426,18 +1352,6 @@ nullableField FieldDecoder {..} =
       allowedPgTypes
     }
 
-{-# INLINE nonNullableRowDec #-}
-nonNullableRowDec :: String -> RowDecoder (Maybe a) -> RowDecoder a
-nonNullableRowDec haskellTypeName rdec =
-  let fromNullable mVal = case mVal of
-        Nothing -> fail $ "Cannot decode SQL null as the Haskell " ++ haskellTypeName ++ " type. Use a `" ++ haskellTypeName ++ "` if you want SQL nulls"
-        Just v -> pure v
-   in RowDecoder
-        { fullRowDecoder = \finfos -> rdec.fullRowDecoder finfos >>= fromNullable,
-          rowColumnsTypeCheck = rdec.rowColumnsTypeCheck,
-          numExpectedColumns = rdec.numExpectedColumns
-        }
-
 instance (FromPgField a) => FromPgField (Maybe a) where
   {-# INLINE fieldDecoder #-}
   fieldDecoder = nullableField fieldDecoder
@@ -1457,36 +1371,6 @@ instance (FromPgField a) => FromPgField (Maybe a) where
         Nothing -> pure Nothing -- Must return Nothing for SQL Nulls
         jv -> pure $ Just jv
 
--- let ffdec = fieldAndValueDecoder @a
--- in
--- RowDecoder
---   { fullRowDecoder = \finfos ->
---       let frd = fieldAndValueDecoder.fullRowDecoder finfos
---        in do
---             -- TODO: We're decoding the field length twice with
---             -- the peek call when the value isn't NULL.
---             -- Maybe we should make `FromPgField`'s new methods
---             -- be two `Parser` objects: one for both length and field
---             -- and another only for the field (but how would that work
---             -- without the length..? It wouldn't.)
---             -- Maybe we do the `Parser (Maybe a)` for `a` types, then.
---             -- We can build a `Parser a` from that with `decodesSqlNullTo`
---             -- and with inlining there's nothing to lose?
---             fieldLen <- Parser.peekInt32BE
---             if fieldLen == (-1)
---               then case fieldDecoder.decodesSqlNullTo of
---                 Left err -> fail err
---                 Right v -> Parser.skip 4 >> pure v
---               else do
---                 Just <$> frd,
---     rowColumnsTypeCheck =
---       let fdec = fieldDecoder @a
---        in \case
---             [singleColInfo] -> [(singleColInfo, fdec.allowedPgTypes singleColInfo)]
---             _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
---     numExpectedColumns = 1
---   }
-
 allowOnlyArrayTypes :: FieldInfo -> Bool
 allowOnlyArrayTypes fieldInfo =
   -- TODO: We could check the elemTypeOid too, but maybe later
@@ -1566,7 +1450,7 @@ instance (FromPgField a) => ProductTypeDecoder (K1 r a) where
   -- coercing instead of fmap reduces memory usage, apparently
   -- by reducing (unnecessary) closures in the final row decoder,
   -- as per looking at GHC Core
-  genRowDecoder = coerce $ singleFieldRowDecoder @a
+  genRowDecoder = coerce $ inlinedSingleFieldRowDecoder @a
 
 genericToPgRow :: forall a. (Generic a, ProductTypeEncoder (Rep a)) => RowEncoder a
 genericToPgRow = contramap from genRowEncoder
diff --git a/hpgsql/src/Hpgsql/Types.hs b/hpgsql/src/Hpgsql/Types.hs
index 8cc68ce..77c2dc1 100644
--- a/hpgsql/src/Hpgsql/Types.hs
+++ b/hpgsql/src/Hpgsql/Types.hs
@@ -40,6 +40,7 @@ instance forall a. (ToPgField a) => ToPgField (PGArray a) where
           }
 
 instance forall a. (FromPgField a) => FromPgField (PGArray a) where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder = PGArray <$> arrayField replicateM fieldDecoder
 
 -- | A way to compose two rows.
@@ -83,13 +84,16 @@ pgJsonByteString :: PgJson -> ByteString
 pgJsonByteString (PgJson bs) = bs
 
 instance FromPgField PgJson where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let -- jsonb has a byte prepended to the contents and json does not
-                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in \bs -> Right $ PgJson $ fixJsonb bs,
+            let
+              -- jsonb has a byte prepended to the contents and json does not
+              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in
+              \bs -> Right $ PgJson $ fixJsonb bs,
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell PgJson type. Use a `Maybe PgJson` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
@@ -102,15 +106,18 @@ newtype Aeson a = Aeson {getAeson :: a}
   deriving newtype (Eq)
 
 instance (FromJSON a) => FromPgField (Aeson a) where
+  {-# INLINE fieldDecoder #-}
   fieldDecoder =
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let -- jsonb has a byte prepended to the contents and json does not
-                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in \bs -> case Aeson.decodeStrict $ fixJsonb bs of
-                  Just v -> Right $ Aeson v
-                  Nothing -> Left "Failed to decode postgres JSON value into your `Aeson a` type. Are you sure it's proper JSON?",
+            let
+              -- jsonb has a byte prepended to the contents and json does not
+              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in
+              \bs -> case Aeson.decodeStrict $ fixJsonb bs of
+                Just v -> Right $ Aeson v
+                Nothing -> Left "Failed to decode postgres JSON value into your `Aeson a` type. Are you sure it's proper JSON?",
         decodesSqlNullTo = Left "Cannot decode SQL null as a Haskell (Aeson a) type. Use a `Maybe (Aeson a)` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }

From 5c7f52f34bc32687e0b21025cf89ff3c8ec7d613 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Wed, 19 Aug 2026 17:09:11 -0300
Subject: [PATCH 18/22] TODOs in the code

---
 hpgsql/src/Hpgsql/Encoding.hs | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index 60cd2e8..6c79e31 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -162,6 +162,7 @@ instance (TypeError (TypeLits.Text "RowDecoder does not have a Monad instance in
 {-# INLINE singleField #-}
 singleField :: FieldDecoder a -> RowDecoder a
 singleField fdec =
+  -- TODO: Float out decodesSqlNullTo to here. Does it make a difference?
   RowDecoder
     { fullRowDecoder = \case
         [singleColInfo] ->
@@ -221,10 +222,16 @@ class FromPgField a where
   -- allocations and thus better performance.
   -- Any implementation of this _must_ return a `Nothing` for a SQL NULL value,
   -- regardless of what `FieldDecoder` would do with a SQL NULL.
-  -- TODO: Move this to inside the FieldDecoder type?
   {-# NOINLINE fieldAndValueDecoder #-}
   fieldAndValueDecoder :: RowDecoder (Maybe a)
   fieldAndValueDecoder =
+    -- TODO: Float out allowedPgTypes? Does it matter at all?
+    -- TODO: This method is.. only useful for the `Scientific` type,
+    -- which can provide a faster row decoder but still needs to know
+    -- the type's OID. Maybe it's useful for our Aeson types too?
+    -- In any case, this class has many methods, and their names should
+    -- better reflect when they're useful and what they do, and `fieldAndValueDecoder`
+    -- might not be doing the best job in the world at that.
     RowDecoder
       { fullRowDecoder =
           case inlinedConstFieldDecoder of
@@ -261,7 +268,8 @@ class FromPgField a where
 
   -- | Semantically equivalent to `singleField fieldDecoder`, but for
   -- some types it can provide a much faster `RowDecoder`. Beware that
-  -- this will produce more code in row decoders.
+  -- using will produce more code in your row decoders, which can affect
+  -- compilation times and binary size.
   {-# INLINE inlinedSingleFieldRowDecoder #-}
   inlinedSingleFieldRowDecoder :: RowDecoder a
   inlinedSingleFieldRowDecoder = case inlinedConstFieldDecoder @a of
@@ -1328,14 +1336,12 @@ instance FromPgField Aeson.Value where
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let
-              -- jsonb has a byte prepended to the contents and json does not
-              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in
-              \case
-                bs -> case Aeson.decodeStrict $ fixJsonb bs of
-                  Just d -> Right d
-                  Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid.",
+            let -- jsonb has a byte prepended to the contents and json does not
+                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in \case
+                  bs -> case Aeson.decodeStrict $ fixJsonb bs of
+                    Just d -> Right d
+                    Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid.",
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }

From 2a2e436aadfedc2305f1056855c4ae2705e3f56b Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Wed, 19 Aug 2026 21:13:47 -0300
Subject: [PATCH 19/22] Trying a specialized notConst method

Types like `Scientific` are not being decoded optimally otherwise, and
they can do better than in the current state
---
 TODO.md                       |   2 +-
 hpgsql/src/Hpgsql/Encoding.hs | 274 +++++++++++++++-------------------
 hpgsql/src/Hpgsql/Types.hs    |  22 ++-
 3 files changed, 133 insertions(+), 165 deletions(-)

diff --git a/TODO.md b/TODO.md
index 6d50f38..7bb0ccb 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,2 +1,2 @@
 - Test both `singleField fieldDecoder` and `singleFieldRowDecoder` for every type in our tests.
-- Some types (the Aeson ones, for example) still don't derive specialized row decoders
+- Some types (the Aeson ones, for example, but more) still don't derive specialized row decoders
diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index 6c79e31..6d0fa9e 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -21,7 +21,7 @@
 -- of fields), check "Hpgsql.Encoding.RowDecoderMonadic".
 module Hpgsql.Encoding
   ( -- * Decoding
-    FromPgField (..),
+    FromPgField (fieldDecoder, inlinedSingleFieldRowDecoder), -- Don't export the other internal perf-oriented methods yet
     FieldDecoder (..), -- TODO: Can we export ctor?
     FieldInfo (..),
     FromPgRow (..),
@@ -121,7 +121,7 @@ data FieldInfo = FieldInfo
 
 -- | A decoder for a single field/column.
 data FieldDecoder a = FieldDecoder
-  { fieldValueDecoder :: FieldInfo -> ByteString -> Either String a, -- TODO: Since this now takes a ByteString (not a Maybe), it could actually be typed `FieldInfo -> Parser a`
+  { fieldValueDecoder :: FieldInfo -> ByteString -> Either String a,
     decodesSqlNullTo :: Either String a,
     allowedPgTypes :: FieldInfo -> Bool
   }
@@ -162,110 +162,76 @@ instance (TypeError (TypeLits.Text "RowDecoder does not have a Monad instance in
 {-# INLINE singleField #-}
 singleField :: FieldDecoder a -> RowDecoder a
 singleField fdec =
-  -- TODO: Float out decodesSqlNullTo to here. Does it make a difference?
-  RowDecoder
-    { fullRowDecoder = \case
-        [singleColInfo] ->
-          let decode = fdec.fieldValueDecoder singleColInfo
-           in do
-                lenNextCol <- fromIntegral <$> Parser.takeInt32BE
-                if lenNextCol >= 0
-                  then do
-                    nextColBs <- Parser.take lenNextCol
-                    case decode nextColBs of
-                      Right v -> pure v
-                      Left err -> fail err
-                  -- This `case` is why we require `fieldAndValueDecoder` to decode
-                  -- SQL NULL into `Nothing`: we do check decodesSqlNullTo.
-                  else case fdec.decodesSqlNullTo of
-                    Right v -> pure v
-                    Left err -> fail err
-        _ -> error "singleField expected a single column OID but got 0 or >1",
-      rowColumnsTypeCheck = \case
-        [singleColInfo] -> [(singleColInfo, fdec.allowedPgTypes singleColInfo)]
-        _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-      numExpectedColumns = 1
-    }
-
-{-# INLINE inlinableRowDecoder #-}
-inlinableRowDecoder :: [Oid] -> Parser.Parser a -> RowDecoder a
-inlinableRowDecoder tyoids p =
-  -- FromPgField instances whose decoders don't care about the OID of the PG type
-  -- being decoded are very dear to us because they allow a very important optimization:
-  -- their row decoders do not care about the `FieldInfo` argument, which
-  -- makes them inlinable by GHC at compile time (FieldInfo is only available
-  -- at run time when the RowDescription message arrives for a given query).
-  -- These are key to produce compiled to code that almost compiles down to
-  -- a bunch of `peek` calls to a single ByteString decoding bytes into
-  -- typed values, to then call the Parser continuation, and repeat.
-  -- The only allocations (I think) when everything is inlined by this are the
-  -- decoded values themselves being boxed and the CPS Parser's ByteStringIdx
-  -- also being passed boxed between continuations (though reading GHC Core
-  -- is something I'm still learning).
-  RowDecoder
-    { fullRowDecoder = const p,
-      rowColumnsTypeCheck = \case
-        [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` tyoids)]
-        _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-      numExpectedColumns = 1
-    }
+  -- This `case` is why we require `fieldAndValueDecoder` to decode
+  -- SQL NULL into `Nothing`: we do check decodesSqlNullTo.
+  let !valueForNull = case fdec.decodesSqlNullTo of
+        Left err -> fail err
+        Right v -> pure v
+      !typeCheck = fdec.allowedPgTypes
+   in RowDecoder
+        { fullRowDecoder = \case
+            [singleColInfo] ->
+              let decode = fdec.fieldValueDecoder singleColInfo
+               in do
+                    lenNextCol <- fromIntegral <$> Parser.takeInt32BE
+                    if lenNextCol >= 0
+                      then do
+                        nextColBs <- Parser.take lenNextCol
+                        case decode nextColBs of
+                          Right v -> pure v
+                          Left err -> fail err
+                      else valueForNull
+            _ -> error "singleField expected a single column OID but got 0 or >1",
+          rowColumnsTypeCheck = \case
+            [singleColInfo] -> [(singleColInfo, typeCheck singleColInfo)]
+            _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+          numExpectedColumns = 1
+        }
 
 class FromPgField a where
   {-# MINIMAL fieldDecoder #-}
   fieldDecoder :: FieldDecoder a
 
-  -- | This should be semantically equivalent to `singleField fieldDecoder`,
-  -- but it can be overridden (and is for base types) to a much faster implementation.
-  -- Using this when deriving your `FromPgRow` instances will increase code size and
-  -- possibly compilation times somewhat, but in some cases it can make row decoders
-  -- compile down to a ByteString-peeking implementation with much fewer
-  -- allocations and thus better performance.
-  -- Any implementation of this _must_ return a `Nothing` for a SQL NULL value,
-  -- regardless of what `FieldDecoder` would do with a SQL NULL.
-  {-# NOINLINE fieldAndValueDecoder #-}
-  fieldAndValueDecoder :: RowDecoder (Maybe a)
-  fieldAndValueDecoder =
-    -- TODO: Float out allowedPgTypes? Does it matter at all?
-    -- TODO: This method is.. only useful for the `Scientific` type,
-    -- which can provide a faster row decoder but still needs to know
-    -- the type's OID. Maybe it's useful for our Aeson types too?
-    -- In any case, this class has many methods, and their names should
-    -- better reflect when they're useful and what they do, and `fieldAndValueDecoder`
-    -- might not be doing the best job in the world at that.
-    RowDecoder
-      { fullRowDecoder =
-          case inlinedConstFieldDecoder of
-            Nothing -> slowerParser
-            Just fd -> const fd,
-        rowColumnsTypeCheck = \case
-          [singleColInfo] -> [(singleColInfo, (fieldDecoder @a).allowedPgTypes singleColInfo)]
-          _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-        numExpectedColumns = 1
-      }
-    where
-      -- slowerParser takes a ByteString and passes it to the
-      -- field decoder.
-      slowerParser = \case
-        [singleColInfo] -> do
-          len <- Parser.takeInt32BE
-          if len == (-1)
-            then pure Nothing
-            else do
-              bs <- Parser.take (fromIntegral len)
-              case fieldDecoder.fieldValueDecoder singleColInfo bs of
-                Left err -> fail err
-                Right v -> pure v
-        _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1"
-
   -- | For types where there is a fast way to decode fields+values
   -- without knowing the OID of the value in the query (of course, the
   -- possible OIDs are still limited by the FieldDecoder's allowed types),
-  -- this can help provide a significant boost to inlined row decoders.
-  -- Define as `Nothing` if this isn't possible.
+  -- defining this can help provide a significant performance boost to inlined row decoders.
+  --
+  -- Any implementation of this _must_ return a `Nothing` for a SQL NULL value,
+  -- regardless of what `FieldDecoder` would do with a SQL NULL.
+  --
+  -- Define this as `Nothing` if implementing it isn't possible.
+  -- This isn't exposed to users yet, but we should recommend they add an INLINE pragma,
+  -- as the method's name suggests.
   {-# INLINE inlinedConstFieldDecoder #-}
   inlinedConstFieldDecoder :: Maybe (Parser.Parser (Maybe a))
   inlinedConstFieldDecoder = Nothing
 
+  -- | For types that can't implement `inlinedConstFieldDecoder`, this is the next
+  -- best thing: also a specialized field+value decoder that can be faster than
+  -- one derived from `fieldDecoder`.
+  --
+  -- Any implementation of this _must_ return a `Nothing` for a SQL NULL value,
+  -- regardless of what `FieldDecoder` would do with a SQL NULL.
+  {-# INLINE notConstFieldDecoder #-}
+  notConstFieldDecoder :: FieldInfo -> Parser.Parser (Maybe a)
+  notConstFieldDecoder =
+    case inlinedConstFieldDecoder of
+      Nothing -> slowerParser
+      Just fd -> const fd
+    where
+      -- slowerParser takes a ByteString and passes it to the
+      -- field decoder.
+      slowerParser singleColInfo = do
+        len <- Parser.takeInt32BE
+        if len == (-1)
+          then pure Nothing
+          else do
+            bs <- Parser.take (fromIntegral len)
+            case fieldDecoder.fieldValueDecoder singleColInfo bs of
+              Left err -> fail err
+              Right v -> pure v
+
   -- | Semantically equivalent to `singleField fieldDecoder`, but for
   -- some types it can provide a much faster `RowDecoder`. Beware that
   -- using will produce more code in your row decoders, which can affect
@@ -275,8 +241,25 @@ class FromPgField a where
   inlinedSingleFieldRowDecoder = case inlinedConstFieldDecoder @a of
     -- This is a class method instead of a top-level function
     -- because the GHC inliner behaves differently when it's a top-level
-    -- function, and benchmarks show it gets worse.
-    Nothing -> singleField fieldDecoder
+    -- function, and benchmarks show this is faster.
+    Nothing ->
+      let !valueForNull = case (fieldDecoder @a).decodesSqlNullTo of
+            Left err -> fail err
+            Right v -> pure v
+          !typeCheck = (fieldDecoder @a).allowedPgTypes
+       in RowDecoder
+            { fullRowDecoder = \case
+                [singleColInfo] -> do
+                  mv <- notConstFieldDecoder singleColInfo
+                  case mv of
+                    Nothing -> valueForNull
+                    Just v -> pure v
+                _ -> error "singleField expected a single column OID but got 0 or >1",
+              rowColumnsTypeCheck = \case
+                [singleColInfo] -> [(singleColInfo, typeCheck singleColInfo)]
+                _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+              numExpectedColumns = 1
+            }
     Just p ->
       -- The strictness and floating out of fieldDecoder-derived
       -- values allows GHC to inline a lot more. For example, `valueForNull`
@@ -318,12 +301,12 @@ compositeTypeDecoder :: forall a. RowDecoder a -> FieldDecoder a
 compositeTypeDecoder (RowDecoder {..}) =
   FieldDecoder
     { fieldValueDecoder = \compositeTypeOid ->
-        let prs = parserForRecord compositeTypeOid.encodingContext <* Parser.endOfInput
+        let !prs = parserForRecord compositeTypeOid.encodingContext <* Parser.endOfInput
          in \bs ->
               case Parser.parseOnly prs bs of
                 Parser.ParseOk v -> Right v
                 Parser.ParseFail err -> Left err,
-      decodesSqlNullTo = Left "TODO: composeTypeDecoder decodesSqlNullTo",
+      decodesSqlNullTo = Left "Got NULL in composite type but it was not allowed",
       allowedPgTypes = const True -- There's no way to enforce a custom type's OID. We only check if it's structurally the same in the parser (same subtypes in same order)
     }
   where
@@ -877,20 +860,6 @@ instance FromPgField () where
         allowedPgTypes = (== voidOid) . fieldTypeOid
       }
 
--- TODO: Inline intRowDecoder into FromPgField? And all others too?
-{-# INLINE intRowDecoder #-}
-intRowDecoder :: Parser.Parser (Maybe Int)
-intRowDecoder = do
-  fieldLen <- Parser.takeInt32BE
-  -- TODO: We're assuming `Int` is always 64 bits, so 64bit CPUs? Is that ok?
-  -- TODO: Is there a way to optimistically assume <=4 bytes and use our custom new parser?
-  case fieldLen of
-    4 -> Just . fromIntegral <$> Parser.takeInt32BE
-    (-1) -> pure Nothing
-    8 -> Just . fromIntegral <$> Parser.takeInt64BE
-    2 -> Just . fromIntegral <$> Parser.takeInt16BE
-    _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
-
 instance FromPgField Int where
   {-# INLINE fieldDecoder #-}
   fieldDecoder =
@@ -903,7 +872,16 @@ instance FromPgField Int where
       }
 
   {-# INLINE inlinedConstFieldDecoder #-}
-  inlinedConstFieldDecoder = Just intRowDecoder
+  inlinedConstFieldDecoder = Just $ do
+    fieldLen <- Parser.takeInt32BE
+    -- TODO: We're assuming `Int` is always 64 bits, so 64bit CPUs? Is that ok?
+    -- TODO: Is there a way to optimistically assume <=4 bytes and use our custom new parser?
+    case fieldLen of
+      4 -> Just . fromIntegral <$> Parser.takeInt32BE
+      (-1) -> pure Nothing
+      8 -> Just . fromIntegral <$> Parser.takeInt64BE
+      2 -> Just . fromIntegral <$> Parser.takeInt16BE
+      _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
 
 instance FromPgField Int16 where
   {-# INLINE fieldDecoder #-}
@@ -916,17 +894,6 @@ instance FromPgField Int16 where
         allowedPgTypes = (== int2Oid) . fieldTypeOid
       }
 
-{-# INLINE int32RowDecoder #-}
-int32RowDecoder :: RowDecoder (Maybe Int32)
-int32RowDecoder =
-  inlinableRowDecoder [int2Oid, int4Oid] $ do
-    fieldLen <- Parser.takeInt32BE
-    case fieldLen of
-      4 -> Just <$> Parser.takeInt32BE
-      (-1) -> pure Nothing
-      2 -> Just . fromIntegral <$> Parser.takeInt16BE
-      _ -> fail "Trying to decode PG int4 but it's not 2 or 4 bytes long"
-
 instance FromPgField Int32 where
   {-# INLINE fieldDecoder #-}
   fieldDecoder =
@@ -935,20 +902,14 @@ instance FromPgField Int32 where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Int32 type. Use a `Maybe Int32`",
         allowedPgTypes = (`elem` [int2Oid, int4Oid]) . fieldTypeOid
       }
-  {-# INLINE fieldAndValueDecoder #-}
-  fieldAndValueDecoder = int32RowDecoder
-
-{-# INLINE int64RowDecoder #-}
-int64RowDecoder :: RowDecoder (Maybe Int64)
-int64RowDecoder =
-  inlinableRowDecoder [int2Oid, int4Oid, int8Oid] $ do
+  {-# INLINE inlinedConstFieldDecoder #-}
+  inlinedConstFieldDecoder = Just $ do
     fieldLen <- Parser.takeInt32BE
     case fieldLen of
-      8 -> Just <$> Parser.takeInt64BE
-      4 -> Just . fromIntegral <$> Parser.takeInt32BE
+      4 -> Just <$> Parser.takeInt32BE
       (-1) -> pure Nothing
       2 -> Just . fromIntegral <$> Parser.takeInt16BE
-      _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
+      _ -> fail "Trying to decode PG int4 but it's not 2 or 4 bytes long"
 
 instance FromPgField Int64 where
   {-# INLINE fieldDecoder #-}
@@ -958,8 +919,15 @@ instance FromPgField Int64 where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Int64 type. Use a `Maybe Int64`",
         allowedPgTypes = (`elem` [int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
-  {-# INLINE fieldAndValueDecoder #-}
-  fieldAndValueDecoder = int64RowDecoder
+  {-# INLINE inlinedConstFieldDecoder #-}
+  inlinedConstFieldDecoder = Just $ do
+    fieldLen <- Parser.takeInt32BE
+    case fieldLen of
+      8 -> Just <$> Parser.takeInt64BE
+      4 -> Just . fromIntegral <$> Parser.takeInt32BE
+      (-1) -> pure Nothing
+      2 -> Just . fromIntegral <$> Parser.takeInt16BE
+      _ -> fail "Trying to decode PG integer but it's not 2, 4 or 8 bytes long"
 
 instance FromPgField Integer where
   {-# INLINE fieldDecoder #-}
@@ -1090,21 +1058,25 @@ instance FromPgField Scientific where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`",
         allowedPgTypes = (`elem` [numericOid, int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
-  {-# INLINE fieldAndValueDecoder #-}
-  fieldAndValueDecoder =
-    RowDecoder
-      { fullRowDecoder = \case
-          [singleColInfo] ->
-            if singleColInfo.fieldTypeOid /= numericOid
-              then
-                fmap (flip scientific 0 . fromIntegral) <$> (fieldAndValueDecoder @Int64).fullRowDecoder [singleColInfo]
-              else numericRowParser
-          _ -> error "singleField expected a single column OID but got 0 or >1",
-        rowColumnsTypeCheck = \case
-          [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` [numericOid, int2Oid, int4Oid, int8Oid])]
-          _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-        numExpectedColumns = 1
-      }
+  {-# INLINE inlinedSingleFieldRowDecoder #-}
+  inlinedSingleFieldRowDecoder =
+    let !int64RowDec = fromMaybe (error "Bug in HPgsql: Int64 does not have an inlinedConstFieldDecoder") $ inlinedConstFieldDecoder @Int64
+     in RowDecoder
+          { fullRowDecoder = \case
+              [singleColInfo] -> do
+                msci <-
+                  if singleColInfo.fieldTypeOid /= numericOid
+                    then fmap (flip scientific 0 . fromIntegral) <$> int64RowDec
+                    else numericRowParser
+                case msci of
+                  Nothing -> fail "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`"
+                  Just sci -> pure sci
+              _ -> error "singleField expected a single column OID but got 0 or >1",
+            rowColumnsTypeCheck = \case
+              [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` [numericOid, int2Oid, int4Oid, int8Oid])]
+              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
+            numExpectedColumns = 1
+          }
 
 instance FromPgField (Ratio Integer) where
   {-# INLINE fieldDecoder #-}
diff --git a/hpgsql/src/Hpgsql/Types.hs b/hpgsql/src/Hpgsql/Types.hs
index 77c2dc1..08a56b6 100644
--- a/hpgsql/src/Hpgsql/Types.hs
+++ b/hpgsql/src/Hpgsql/Types.hs
@@ -64,7 +64,7 @@ instance forall a b. (ToPgRow a, ToPgRow b) => ToPgRow (a :. b) where
 instance (FromPgRow a, FromPgRow b) => FromPgRow (a :. b) where
   rowDecoder = (:.) <$> rowDecoder <*> rowDecoder
 
--- | A JSON type that does not incur the costs of deserializing
+-- | A JSON type that does not incur the costs of JSON/aeson deserializing
 -- in its `FromPgField` instance because it assumes postgres only generates
 -- valid JSON. Useful for extra performance if its opaqueness is not a problem.
 -- Although it does have a `toJSON` method, using it will incur a
@@ -89,11 +89,9 @@ instance FromPgField PgJson where
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let
-              -- jsonb has a byte prepended to the contents and json does not
-              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in
-              \bs -> Right $ PgJson $ fixJsonb bs,
+            let -- jsonb has a byte prepended to the contents and json does not
+                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in \bs -> Right $ PgJson $ fixJsonb bs,
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell PgJson type. Use a `Maybe PgJson` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
@@ -111,13 +109,11 @@ instance (FromJSON a) => FromPgField (Aeson a) where
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let
-              -- jsonb has a byte prepended to the contents and json does not
-              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in
-              \bs -> case Aeson.decodeStrict $ fixJsonb bs of
-                Just v -> Right $ Aeson v
-                Nothing -> Left "Failed to decode postgres JSON value into your `Aeson a` type. Are you sure it's proper JSON?",
+            let -- jsonb has a byte prepended to the contents and json does not
+                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in \bs -> case Aeson.decodeStrict $ fixJsonb bs of
+                  Just v -> Right $ Aeson v
+                  Nothing -> Left "Failed to decode postgres JSON value into your `Aeson a` type. Are you sure it's proper JSON?",
         decodesSqlNullTo = Left "Cannot decode SQL null as a Haskell (Aeson a) type. Use a `Maybe (Aeson a)` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }

From 9c02dc1bb70dccf235d0f93a117bc56b9057c12a Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Thu, 20 Aug 2026 14:31:31 -0300
Subject: [PATCH 20/22] Do the JSON types, but hpgsql-simple-compat will break

Becase we don't have a FieldInfo when decoding NULL anymore. Not sure
what we can do.
---
 Runfile                                       |  6 +--
 TODO.md                                       |  1 +
 .../Database/PostgreSQL/Simple/FromField.hs   | 10 +++--
 .../Database/PostgreSQL/Simple/HpgsqlUtils.hs | 21 +++++++---
 hpgsql/src/Hpgsql/Encoding.hs                 | 38 +++++++++----------
 hpgsql/src/Hpgsql/Types.hs                    | 21 +++++++++-
 6 files changed, 64 insertions(+), 33 deletions(-)

diff --git a/Runfile b/Runfile
index 0a791b9..406b180 100644
--- a/Runfile
+++ b/Runfile
@@ -73,13 +73,13 @@ tests:
         if [ -n "$NIX" ]; then
             nix-build --no-out-link -A "testsPg${pg}" --argstr hspecArgs "$TARGS"
         else
-            cabal build hpgsql-tests # hpgsql-simple-compat-tests
+            cabal build hpgsql-tests hpgsql-simple-compat-tests
             nix-shell -A "shellPg${pg}" ./default.nix --run "./scripts/run-tests-db-internal.sh $TARGS"
         fi
     done
 
-    # echo "--- Running hlint"
-    # hlint .
+    echo "--- Running hlint"
+    hlint .
 
 ##
 # Runs tests 100 times, reporting how many passed and how many failed.
diff --git a/TODO.md b/TODO.md
index 7bb0ccb..95ea5db 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,2 +1,3 @@
 - Test both `singleField fieldDecoder` and `singleFieldRowDecoder` for every type in our tests.
 - Some types (the Aeson ones, for example, but more) still don't derive specialized row decoders
+- "Oh no! No colInfo here.. what do we do!?" in hpgsql-simple-compat. This might require a big rethinking of things..
diff --git a/hpgsql-simple-compat/src/Database/PostgreSQL/Simple/FromField.hs b/hpgsql-simple-compat/src/Database/PostgreSQL/Simple/FromField.hs
index 3a2b83b..42e124e 100644
--- a/hpgsql-simple-compat/src/Database/PostgreSQL/Simple/FromField.hs
+++ b/hpgsql-simple-compat/src/Database/PostgreSQL/Simple/FromField.hs
@@ -177,9 +177,13 @@ class FromField a where
     let dec = Hpgsql.fieldDecoder
      in \f ->
           if Hpgsql.allowedPgTypes dec f
-            then \mbs -> Conversion $ \_encCtx -> case Hpgsql.fieldValueDecoder dec f mbs of
-              Right v -> Ok v
-              Left err -> Errors [toException $ userError err]
+            then \mbs -> Conversion $ \_encCtx -> case mbs of
+              Nothing -> case dec.decodesSqlNullTo of
+                Left err -> Errors [toException $ userError err]
+                Right v -> Ok v
+              Just bs -> case Hpgsql.fieldValueDecoder dec f bs of
+                Right v -> Ok v
+                Left err -> Errors [toException $ userError err]
             else \_ -> Conversion $ \_encCtx -> Errors [toException $ userError "Invalid type OID for FromField instance"]
 
 instance FromField ()
diff --git a/hpgsql-simple-compat/src/Database/PostgreSQL/Simple/HpgsqlUtils.hs b/hpgsql-simple-compat/src/Database/PostgreSQL/Simple/HpgsqlUtils.hs
index 63a2898..cf42a1d 100644
--- a/hpgsql-simple-compat/src/Database/PostgreSQL/Simple/HpgsqlUtils.hs
+++ b/hpgsql-simple-compat/src/Database/PostgreSQL/Simple/HpgsqlUtils.hs
@@ -95,18 +95,29 @@ type FieldParser a = Field -> Maybe ByteString -> Conversion a
 toHpgsqlFieldDecoder :: FieldParser a -> FieldDecoder a
 toHpgsqlFieldDecoder fp =
   FieldDecoder
-    { fieldValueDecoder = \colInfo mbs ->
-        let valConv = fp colInfo mbs
+    { fieldValueDecoder = \colInfo bs ->
+        let valConv = fp colInfo (Just bs)
          in case runConversion valConv colInfo.encodingContext of
               Ok v -> Right v
               Errors errs -> Left (show errs),
+      decodesSqlNullTo =
+        let valConv = fp (error "Oh no! No colInfo here.. what do we do!?") Nothing
+            encCtx = error "We could fake an EncodingContext, at least. TODO."
+         in case runConversion valConv encCtx of
+              Ok v -> Right v
+              Errors errs -> Left (show errs),
       allowedPgTypes = const True -- No way to check if types are valid ahead of time
     }
 
 fromHpgsqlFieldDecoder :: FieldDecoder a -> FieldParser a
-fromHpgsqlFieldDecoder dec = \f mbs -> Conversion $ \_encCtx -> case dec.fieldValueDecoder f mbs of
-  Right v -> Ok v
-  Left err -> Errors [toException $ userError $ show err]
+fromHpgsqlFieldDecoder dec = \f mbs -> Conversion $ \_encCtx ->
+  case mbs of
+    Nothing -> case dec.decodesSqlNullTo of
+      Left err -> Errors [toException $ userError $ show err]
+      Right v -> Ok v
+    Just bs -> case dec.fieldValueDecoder f bs of
+      Right v -> Ok v
+      Left err -> Errors [toException $ userError $ show err]
 
 -- | Given a Hpgsql query, returns the text format with question marks
 -- for query arguments and a row object. With both, you can call
diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index 6d0fa9e..919278e 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -21,7 +21,7 @@
 -- of fields), check "Hpgsql.Encoding.RowDecoderMonadic".
 module Hpgsql.Encoding
   ( -- * Decoding
-    FromPgField (fieldDecoder, inlinedSingleFieldRowDecoder), -- Don't export the other internal perf-oriented methods yet
+    FromPgField (..), -- We export the other internal perf-oriented methods, which isn't great because we may want to change them
     FieldDecoder (..), -- TODO: Can we export ctor?
     FieldInfo (..),
     FromPgRow (..),
@@ -207,8 +207,9 @@ class FromPgField a where
   inlinedConstFieldDecoder :: Maybe (Parser.Parser (Maybe a))
   inlinedConstFieldDecoder = Nothing
 
-  -- | For types that can't implement `inlinedConstFieldDecoder`, this is the next
-  -- best thing: also a specialized field+value decoder that can be faster than
+  -- | For types that can't implement `inlinedConstFieldDecoder` because they
+  -- need to know the value's OID for decoding, this is the next best thing:
+  -- also a specialized field+value decoder that can be faster than the
   -- one derived from `fieldDecoder`.
   --
   -- Any implementation of this _must_ return a `Nothing` for a SQL NULL value,
@@ -1058,25 +1059,13 @@ instance FromPgField Scientific where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`",
         allowedPgTypes = (`elem` [numericOid, int2Oid, int4Oid, int8Oid]) . fieldTypeOid
       }
-  {-# INLINE inlinedSingleFieldRowDecoder #-}
-  inlinedSingleFieldRowDecoder =
+  {-# INLINE notConstFieldDecoder #-}
+  notConstFieldDecoder =
     let !int64RowDec = fromMaybe (error "Bug in HPgsql: Int64 does not have an inlinedConstFieldDecoder") $ inlinedConstFieldDecoder @Int64
-     in RowDecoder
-          { fullRowDecoder = \case
-              [singleColInfo] -> do
-                msci <-
-                  if singleColInfo.fieldTypeOid /= numericOid
-                    then fmap (flip scientific 0 . fromIntegral) <$> int64RowDec
-                    else numericRowParser
-                case msci of
-                  Nothing -> fail "Cannot decode SQL null as the Haskell Scientific type. Use a `Maybe Scientific`"
-                  Just sci -> pure sci
-              _ -> error "singleField expected a single column OID but got 0 or >1",
-            rowColumnsTypeCheck = \case
-              [singleColInfo] -> [(singleColInfo, singleColInfo.fieldTypeOid `elem` [numericOid, int2Oid, int4Oid, int8Oid])]
-              _ -> error "singleField's rowColumnsTypeCheck expected a single column OID but got 0 or >1",
-            numExpectedColumns = 1
-          }
+     in \singleColInfo ->
+          if singleColInfo.fieldTypeOid /= numericOid
+            then fmap (flip scientific 0 . fromIntegral) <$> int64RowDec
+            else numericRowParser
 
 instance FromPgField (Ratio Integer) where
   {-# INLINE fieldDecoder #-}
@@ -1334,6 +1323,13 @@ instance (FromPgField a) => FromPgField (Maybe a) where
   {-# INLINE fieldDecoder #-}
   fieldDecoder = nullableField fieldDecoder
 
+  {-# INLINE notConstFieldDecoder #-}
+  notConstFieldDecoder finfo = do
+    mv <- notConstFieldDecoder @a finfo
+    case mv of
+      Nothing -> pure Nothing
+      jv -> pure $ Just jv
+
   {-# INLINE inlinedConstFieldDecoder #-}
   -- \| For types where there is a fast way to decode fields+values
   -- without knowing the OID of the value in the query (of course, the
diff --git a/hpgsql/src/Hpgsql/Types.hs b/hpgsql/src/Hpgsql/Types.hs
index 08a56b6..2fc1dbe 100644
--- a/hpgsql/src/Hpgsql/Types.hs
+++ b/hpgsql/src/Hpgsql/Types.hs
@@ -20,6 +20,7 @@ import Data.Tuple.Only (Only (..))
 import Data.Typeable (Proxy (..))
 import Hpgsql.Builder (BinaryField (..))
 import Hpgsql.Encoding (FieldDecoder (..), FieldEncoder (..), FieldInfo (..), FromPgField (..), FromPgRow (..), RowEncoder (..), ToPgField (..), ToPgRow (..), arrayField, toPgVectorField)
+import qualified Hpgsql.SimpleParser as Parser
 import Hpgsql.TypeInfo (EncodingContext (..), TypeInfo (..), jsonOid, jsonbOid, lookupTypeByOid)
 
 -- | Encodes a Haskell list as a postgres array. You can also use `Vector` if you prefer.
@@ -95,6 +96,16 @@ instance FromPgField PgJson where
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell PgJson type. Use a `Maybe PgJson` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
+  {-# INLINE notConstFieldDecoder #-}
+  notConstFieldDecoder finfo = do
+    len <- fromIntegral <$> Parser.takeInt32BE
+    if len == (-1)
+      then pure Nothing
+      else
+        fmap (Just . PgJson) $
+          if finfo.fieldTypeOid == jsonbOid
+            then Parser.skip 1 >> Parser.take (len - 1)
+            else Parser.take len
 
 -- | A newtype wrapper to decode a JSON value with Aeson
 -- into your type (from either json or jsonb), and to encode
@@ -113,10 +124,18 @@ instance (FromJSON a) => FromPgField (Aeson a) where
                 !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
              in \bs -> case Aeson.decodeStrict $ fixJsonb bs of
                   Just v -> Right $ Aeson v
-                  Nothing -> Left "Failed to decode postgres JSON value into your `Aeson a` type. Are you sure it's proper JSON?",
+                  Nothing -> Left "Failed to decode the postgres JSON value into your `Aeson a` type with aeson",
         decodesSqlNullTo = Left "Cannot decode SQL null as a Haskell (Aeson a) type. Use a `Maybe (Aeson a)` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
+  {-# INLINE notConstFieldDecoder #-}
+  notConstFieldDecoder finfo =
+    notConstFieldDecoder finfo >>= \case
+      Nothing -> pure Nothing
+      Just (PgJson jsonBs) ->
+        case Aeson.decodeStrict jsonBs of
+          Just v -> pure $ Just $ Aeson v
+          Nothing -> fail "Failed to decode the postgres JSON value into your `Aeson a` type with aeson"
 
 instance (ToJSON a) => ToPgField (Aeson a) where
   fieldEncoder =

From c0c7003364b6b5a45e07ced272372fc0d0242c64 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Sun, 23 Aug 2026 12:12:47 -0300
Subject: [PATCH 21/22] Post-rebase fixes

---
 hpgsql/src/Hpgsql/Encoding.hs                 | 16 ++++++----
 .../src/Hpgsql/Encoding/BinarySerializer.hs   | 32 ++++++++-----------
 hpgsql/src/Hpgsql/SimpleParser.hs             |  6 ++--
 hpgsql/src/Hpgsql/Types.hs                    | 20 +++++++-----
 4 files changed, 38 insertions(+), 36 deletions(-)

diff --git a/hpgsql/src/Hpgsql/Encoding.hs b/hpgsql/src/Hpgsql/Encoding.hs
index 919278e..194d76f 100644
--- a/hpgsql/src/Hpgsql/Encoding.hs
+++ b/hpgsql/src/Hpgsql/Encoding.hs
@@ -1076,7 +1076,7 @@ binaryTrue = BinSer.encodePgBoolean True
 
 {-# INLINE boolRowDecoder #-}
 boolRowDecoder :: Parser.Parser (Maybe Bool)
-boolRowDecoder = fmap (== 1) <$> Parser.parsePgFieldWithAtMost4Bytes BinSer.TypeSize1
+boolRowDecoder = fmap (== 1) <$> Parser.parsePgFieldWithAtMost4Bytes BinSer.CWord8
 
 instance FromPgField Bool where
   {-# INLINE fieldDecoder #-}
@@ -1297,12 +1297,14 @@ instance FromPgField Aeson.Value where
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let -- jsonb has a byte prepended to the contents and json does not
-                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in \case
-                  bs -> case Aeson.decodeStrict $ fixJsonb bs of
-                    Just d -> Right d
-                    Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid.",
+            let
+              -- jsonb has a byte prepended to the contents and json does not
+              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in
+              \case
+                bs -> case Aeson.decodeStrict $ fixJsonb bs of
+                  Just d -> Right d
+                  Nothing -> Left "Bug in Hpgsql. Postgres produced a json or jsonb value that Aeson does not consider valid.",
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell Aeson.Value type. Use a `Maybe Aeson.Value` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
diff --git a/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs b/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
index a1274b2..a213084 100644
--- a/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
+++ b/hpgsql/src/Hpgsql/Encoding/BinarySerializer.hs
@@ -64,14 +64,14 @@ fromBigEndian16 = Prelude.id
 fromBigEndian16 = byteSwap16
 #endif
 
-data CoolWordDec a where
-  CWord8 :: CoolWordDec Word8
-  CWord16 :: CoolWordDec Word16
-  CWord32 :: CoolWordDec Word32
-  CWord64 :: CoolWordDec Word64
+data WordDecoding a where
+  CWord8 :: WordDecoding Word8
+  CWord16 :: WordDecoding Word16
+  CWord32 :: WordDecoding Word32
+  CWord64 :: WordDecoding Word64
 
 {-# INLINE decodeWord #-}
-decodeWord :: CoolWordDec a -> ByteStringIdx -> ByteString -> (a -> a) -> Either String a
+decodeWord :: WordDecoding a -> ByteStringIdx -> ByteString -> (a -> a) -> Either String a
 decodeWord wdec idx (InternalBS.BS bytesPtr len) endianConvert =
   case wdec of
     CWord8 -> if len < 1 + idx.idx then Left "Less than enough bytes to decode" else Right $ endianConvert $ unsafeDupablePerformIO $ withForeignPtr bytesPtr $ \ptr -> peekByteOff (coerce ptr) idx.idx
@@ -102,11 +102,11 @@ decodeWord8 idx bs = decodeWord CWord8 idx bs Prelude.id
 
 {-# INLINE decodeWord32BE #-}
 decodeWord32BE :: ByteStringIdx -> ByteString -> Either String Word32
-decodeWord32BE idx bs = unsafeDecodeWord idx bs 4 fromBigEndian32
+decodeWord32BE idx bs = decodeWord CWord32 idx bs fromBigEndian32
 
 {-# INLINE decodeWord64BE #-}
 decodeWord64BE :: ByteStringIdx -> ByteString -> Either String Word64
-decodeWord64BE idx bs = unsafeDecodeWord idx bs 8 fromBigEndian64
+decodeWord64BE idx bs = decodeWord CWord64 idx bs fromBigEndian64
 
 {-# INLINE decodeInt32BE #-}
 decodeInt32BE :: ByteStringIdx -> ByteString -> Either String Int32
@@ -178,11 +178,6 @@ decodeDataRow idx bs@(InternalBS.BS _bytesPtr len) =
       | len >= 1 + lenFullMsg + idx.idx = Right $ ByteStringIdx $ 1 + lenFullMsg + idx.idx
       | otherwise = Left "Less than enough bytes to decode a full DataRow"
 
-data WordDecoding a where
-  TypeSize1 :: WordDecoding Word8
-  TypeSize2 :: WordDecoding Word16
-  TypeSize4 :: WordDecoding Word32
-
 {-# INLINE decodePgFieldWithAtMost4Bytes #-}
 
 -- | A specialized decoder that decoders a query result's
@@ -195,9 +190,10 @@ data WordDecoding a where
 decodePgFieldWithAtMost4Bytes :: forall a. (Storable a, Integral a) => WordDecoding a -> ByteStringIdx -> ByteString -> Either String (Maybe a, ByteStringIdx)
 decodePgFieldWithAtMost4Bytes wdec =
   let (pgTypeSize, endianSwap, valueMask :: Word64) = case wdec of
-        TypeSize1 -> (1, Prelude.id, 0b00000000_00000000_00000000_00000000_11111111_00000000_00000000_00000000)
-        TypeSize2 -> (2, fromBigEndian16, 0b00000000_00000000_00000000_00000000_11111111_11111111_00000000_00000000)
-        TypeSize4 -> (4, fromBigEndian32, 0b00000000_00000000_00000000_00000000_11111111_11111111_11111111_11111111)
+        CWord8 -> (1, Prelude.id, 0b00000000_00000000_00000000_00000000_11111111_00000000_00000000_00000000)
+        CWord16 -> (2, fromBigEndian16, 0b00000000_00000000_00000000_00000000_11111111_11111111_00000000_00000000)
+        CWord32 -> (4, fromBigEndian32, 0b00000000_00000000_00000000_00000000_11111111_11111111_11111111_11111111)
+        CWord64 -> error "Cannot try to decode 64bit words with this function"
       valueShift :: Int = 8 * (4 - pgTypeSize)
    in \idx bs ->
         -- We try the most optimistic case first:
@@ -205,7 +201,7 @@ decodePgFieldWithAtMost4Bytes wdec =
         -- - Null int32 followed by at least one other field (not the last field in the row)
         -- - Shorter types (int16, bool) followed by at least one other field (not the last field in the row)
         -- In all the cases above, there are at least 8 bytes in the row, so our decoding into a Word64 will succeed.
-        case unsafeDecodeWord idx bs 8 fromBigEndian64 of
+        case decodeWord CWord64 idx bs fromBigEndian64 of
           Right (w64 :: Word64) ->
             let fieldLenW64 :: Word64 = flip unsafeShiftR 32 $ w64 .&. 0b11111111_11111111_11111111_11111111_00000000_00000000_00000000_00000000
                 fieldIfNotNull :: a = fromIntegral $ unsafeShiftR (w64 .&. valueMask) valueShift
@@ -225,6 +221,6 @@ decodePgFieldWithAtMost4Bytes wdec =
             if lenField >= 0
               then do
                 -- peek after the next 4 bytes for @a
-                fieldValue <- unsafeDecodeWord (idx + 4) bs (fromIntegral pgTypeSize) endianSwap
+                fieldValue <- decodeWord wdec (idx + 4) bs endianSwap
                 Right (Just fieldValue, idx + 4 + fromIntegral lenField)
               else Right (Nothing, idx + 4)
diff --git a/hpgsql/src/Hpgsql/SimpleParser.hs b/hpgsql/src/Hpgsql/SimpleParser.hs
index 5c9e499..d0bc4ae 100644
--- a/hpgsql/src/Hpgsql/SimpleParser.hs
+++ b/hpgsql/src/Hpgsql/SimpleParser.hs
@@ -143,7 +143,7 @@ takeInt16BE = Parser $ \idx bs kf ks ->
 -- an Int16 in a row.
 takeInt16BEWithFieldLength :: Parser (Maybe Int16)
 takeInt16BEWithFieldLength = do
-  mi16 <- parsePgFieldWithAtMost4Bytes BinSer.TypeSize2
+  mi16 <- parsePgFieldWithAtMost4Bytes BinSer.CWord16
   pure $ fromIntegral <$> mi16
 
 {-# INLINE takeInt32BE #-}
@@ -166,7 +166,7 @@ peekInt32BE = Parser $ \idx bs kf ks ->
 -- an Int32 in a row.
 takeInt32BEWithFieldLength :: Parser (Maybe Int32)
 takeInt32BEWithFieldLength = do
-  mi32 <- parsePgFieldWithAtMost4Bytes BinSer.TypeSize4
+  mi32 <- parsePgFieldWithAtMost4Bytes BinSer.CWord32
   pure $ fromIntegral <$> mi32
 
 {-# INLINE takeFloatBEWithFieldLength #-}
@@ -175,7 +175,7 @@ takeInt32BEWithFieldLength = do
 -- a Float in a row.
 takeFloatBEWithFieldLength :: Parser (Maybe Float)
 takeFloatBEWithFieldLength = do
-  mf <- parsePgFieldWithAtMost4Bytes BinSer.TypeSize4
+  mf <- parsePgFieldWithAtMost4Bytes BinSer.CWord32
   pure $ castWord32ToFloat <$> mf
 
 {-# INLINE takeFloatBE #-}
diff --git a/hpgsql/src/Hpgsql/Types.hs b/hpgsql/src/Hpgsql/Types.hs
index 2fc1dbe..7a67b51 100644
--- a/hpgsql/src/Hpgsql/Types.hs
+++ b/hpgsql/src/Hpgsql/Types.hs
@@ -90,9 +90,11 @@ instance FromPgField PgJson where
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let -- jsonb has a byte prepended to the contents and json does not
-                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in \bs -> Right $ PgJson $ fixJsonb bs,
+            let
+              -- jsonb has a byte prepended to the contents and json does not
+              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in
+              \bs -> Right $ PgJson $ fixJsonb bs,
         decodesSqlNullTo = Left "Cannot decode SQL null as the Haskell PgJson type. Use a `Maybe PgJson` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }
@@ -120,11 +122,13 @@ instance (FromJSON a) => FromPgField (Aeson a) where
     FieldDecoder
       { fieldValueDecoder =
           \FieldInfo {fieldTypeOid} ->
-            let -- jsonb has a byte prepended to the contents and json does not
-                !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
-             in \bs -> case Aeson.decodeStrict $ fixJsonb bs of
-                  Just v -> Right $ Aeson v
-                  Nothing -> Left "Failed to decode the postgres JSON value into your `Aeson a` type with aeson",
+            let
+              -- jsonb has a byte prepended to the contents and json does not
+              !fixJsonb = if fieldTypeOid == jsonbOid then BS.drop 1 else Prelude.id
+             in
+              \bs -> case Aeson.decodeStrict $ fixJsonb bs of
+                Just v -> Right $ Aeson v
+                Nothing -> Left "Failed to decode the postgres JSON value into your `Aeson a` type with aeson",
         decodesSqlNullTo = Left "Cannot decode SQL null as a Haskell (Aeson a) type. Use a `Maybe (Aeson a)` if you want SQL nulls",
         allowedPgTypes = (`elem` [jsonOid, jsonbOid]) . fieldTypeOid
       }

From 4861e73d1931b6bbd2ded70b50478f28ee7f3830 Mon Sep 17 00:00:00 2001
From: Marcelo Zabani <mzabani@gmail.com>
Date: Sun, 23 Aug 2026 12:16:02 -0300
Subject: [PATCH 22/22] Update TODO

---
 TODO.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/TODO.md b/TODO.md
index 95ea5db..82fd6ab 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,3 +1,4 @@
 - Test both `singleField fieldDecoder` and `singleFieldRowDecoder` for every type in our tests.
 - Some types (the Aeson ones, for example, but more) still don't derive specialized row decoders
 - "Oh no! No colInfo here.. what do we do!?" in hpgsql-simple-compat. This might require a big rethinking of things..
+- Double-check which row encoders we want to use the inlined versions for and which we don't. Tuples?