PostgreSQL: force wire-level BEGIN when starting a transaction

src/java/arjdbc/postgresql/PostgreSQLRubyJdbcConnection.java

@@ -173,6 +173,21 @@ protected final IRubyObject beginTransaction(final ThreadContext context, final
         // NOTE: only reversed order - just to ~ match how Rails does it :
         /* if ( connection.getAutoCommit() ) */ connection.setAutoCommit(false);
         if ( isolation != null ) setTransactionIsolation(context, connection, isolation);
+        // pgjdbc's setAutoCommit(false) only flips a client-side flag; the
+        // wire-level BEGIN is deferred until the next statement is sent
+        // (QueryExecutorImpl#sendQueryPreamble). Rails 7.1+ lazy transactions
+        // assume begin_db_transaction opens a server-side transaction once
+        // materialized, so flush the deferred BEGIN now with a no-op statement
+        // - pgjdbc prepends BEGIN to it in the same round-trip, matching the
+        // cost of pg's exec("BEGIN").
+        Statement statement = null;
+        try {
+            statement = connection.createStatement();
+            statement.execute("SELECT 1");
+        }
+        finally {
+            close(statement);
+        }
         return context.nil;
     }
 

test/db/postgresql/transaction_test.rb

@@ -8,6 +8,20 @@ def test_supports_savepoints
     assert_true ActiveRecord::Base.connection.supports_savepoints?
   end
 
+  # pgjdbc defers the wire-level BEGIN until the next statement is sent, but
+  # AR 7.1+ lazy transactions expect begin_db_transaction to actually open a
+  # server-side transaction (e.g. materialize_transactions), so it must flush
+  # the deferred BEGIN right away.
+  def test_begin_db_transaction_opens_transaction_on_the_server
+    connection = ActiveRecord::Base.connection
+    assert_equal 'IDLE', connection.jdbc_connection(true).transaction_state.to_s
+
+    connection.begin_db_transaction
+    assert_equal 'OPEN', connection.jdbc_connection(true).transaction_state.to_s
+  ensure
+    connection.rollback_db_transaction
+  end
+
   # @override
   def test_releasing_named_savepoints
     omit 'savepoins not supported' unless @supports_savepoints