From 772e6b08c8d36e78f111af346a9804fd0e7771b7 Mon Sep 17 00:00:00 2001
From: npub1t2tgm7d8f995uqvmnm8h88sg3wnpp9a5xysjf6dg3tjmgt3ltulqdp8ehr
 <5a968df9a7494b4e019b9ecf739e088ba61097b4312124e9a88ae5b42e3f5f3e@sprout-oss.stage.blox.sqprod.co>
Date: Sun, 12 Jul 2026 21:47:18 -0400
Subject: [PATCH] perf(db): index thread reply keyset pagination

Co-authored-by: npub1t2tgm7d8f995uqvmnm8h88sg3wnpp9a5xysjf6dg3tjmgt3ltulqdp8ehr <5a968df9a7494b4e019b9ecf739e088ba61097b4312124e9a88ae5b42e3f5f3e@sprout-oss.stage.blox.sqprod.co>
Signed-off-by: npub1t2tgm7d8f995uqvmnm8h88sg3wnpp9a5xysjf6dg3tjmgt3ltulqdp8ehr <5a968df9a7494b4e019b9ecf739e088ba61097b4312124e9a88ae5b42e3f5f3e@sprout-oss.stage.blox.sqprod.co>
---
 crates/buzz-db/src/migration.rs               | 19 ++++++++++++++++++-
 .../0007_thread_metadata_keyset_index.sql     |  9 +++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 migrations/0007_thread_metadata_keyset_index.sql

diff --git a/crates/buzz-db/src/migration.rs b/crates/buzz-db/src/migration.rs
index b255e79cbe..61c3e9b92c 100644
--- a/crates/buzz-db/src/migration.rs
+++ b/crates/buzz-db/src/migration.rs
@@ -471,7 +471,7 @@ mod tests {
         let mut migrations: Vec<_> = MIGRATOR.iter().collect();
         migrations.sort_by_key(|migration| migration.version);
 
-        assert_eq!(migrations.len(), 6);
+        assert_eq!(migrations.len(), 7);
         assert_eq!(migrations[0].version, 1);
         assert_eq!(&*migrations[0].description, "initial schema");
         assert!(migrations[0]
@@ -555,6 +555,23 @@ mod tests {
             );
         }
         assert!(!migrations[0].sql.as_str().contains("moderation_reports"));
+
+        // Thread subtree pagination uses the full tenant/root/keyset tuple. Keep
+        // the covering index additive so brownfield migration checksums remain
+        // stable.
+        assert_eq!(migrations[6].version, 7);
+        assert!(migrations[6]
+            .sql
+            .as_str()
+            .contains("CREATE INDEX idx_thread_metadata_root_keyset"));
+        assert!(migrations[6]
+            .sql
+            .as_str()
+            .contains("(community_id, root_event_id, event_created_at, event_id)"));
+        assert!(!migrations[0]
+            .sql
+            .as_str()
+            .contains("idx_thread_metadata_root_keyset"));
     }
 
     #[test]
diff --git a/migrations/0007_thread_metadata_keyset_index.sql b/migrations/0007_thread_metadata_keyset_index.sql
new file mode 100644
index 0000000000..219c706b01
--- /dev/null
+++ b/migrations/0007_thread_metadata_keyset_index.sql
@@ -0,0 +1,9 @@
+-- ── Thread reply keyset pagination index ─────────────────────────────────────
+-- Thread subtree reads filter by tenant and root, then page in ascending
+-- (event_created_at, event_id) order. Include the keyset columns so PostgreSQL
+-- can satisfy both the filter and ordering from one index scan.
+--
+-- Additive migration: previously applied files must not change checksum.
+
+CREATE INDEX idx_thread_metadata_root_keyset
+    ON thread_metadata (community_id, root_event_id, event_created_at, event_id);