sql: implement pg_catalog.pg_prepared_xacts

Informs #22329.

`pg_catalog.pg_prepared_xacts` reads from `system.prepared_transactions`.

Release note: None

pkg/cli/clisqlshell/testdata/describe

@@ -563,7 +563,7 @@ pg_catalog,pg_policies,table,node,permanent,prefix,pg_policies was created for c
 pg_catalog,pg_policy,table,node,permanent,prefix,pg_policy was created for compatibility and is currently unimplemented
 pg_catalog,pg_prepared_statements,table,node,permanent,prefix,"prepared statements
 https://www.postgresql.org/docs/9.6/view-pg-prepared-statements.html"
-pg_catalog,pg_prepared_xacts,table,node,permanent,prefix,"prepared transactions (empty - feature does not exist)
+pg_catalog,pg_prepared_xacts,table,node,permanent,prefix,"prepared transactions
 https://www.postgresql.org/docs/9.6/view-pg-prepared-xacts.html"
 pg_catalog,pg_proc,table,node,permanent,prefix,"built-in functions (incomplete)
 https://www.postgresql.org/docs/16/catalog-pg-proc.html"

pkg/sql/logictest/testdata/logic_test/crdb_internal

@@ -126,7 +126,7 @@ pg_partitioned_table             true
 pg_policies                      true
 pg_policy                        true
 pg_prepared_statements           false
-pg_prepared_xacts                true
+pg_prepared_xacts                false
 pg_proc                          false
 pg_publication                   true
 pg_publication_rel               true

pkg/sql/logictest/testdata/logic_test/two_phase_commit

@@ -39,10 +39,10 @@ read-only  root  test
 
 # Verify the prepared transaction is visible in the pg_catalog.pg_prepared_xacts
 # table.
-# TODO(nvanbenschoten): implement.
 query ITTT
 SELECT transaction, gid, owner, database FROM pg_catalog.pg_prepared_xacts
 ----
+0  read-only  root  test
 
 # Commit the read-only prepared transaction.
 statement ok
@@ -279,10 +279,11 @@ SELECT global_id, owner, database FROM system.prepared_transactions
 read-write-root      root      test
 read-write-testuser  testuser  test
 
-# TODO(nvanbenschoten): implement.
 query ITTT rowsort
 SELECT transaction, gid, owner, database FROM pg_catalog.pg_prepared_xacts
 ----
+0  read-write-root      root      test
+0  read-write-testuser  testuser  test
 
 statement ok
 ROLLBACK PREPARED 'read-write-root'

pkg/sql/pg_catalog.go

@@ -2419,13 +2419,48 @@ https://www.postgresql.org/docs/9.5/catalog-pg-operator.html`,
 }
 
 var pgCatalogPreparedXactsTable = virtualSchemaTable{
-	comment: `prepared transactions (empty - feature does not exist)
+	comment: `prepared transactions
 https://www.postgresql.org/docs/9.6/view-pg-prepared-xacts.html`,
 	schema: vtable.PGCatalogPreparedXacts,
 	populate: func(ctx context.Context, p *planner, dbContext catalog.DatabaseDescriptor, addRow func(...tree.Datum) error) error {
+		rows, err := p.InternalSQLTxn().QueryBufferedEx(
+			ctx,
+			"select-prepared-transactions",
+			p.Txn(),
+			sessiondata.NodeUserSessionDataOverride,
+			`SELECT global_id, prepared, owner, database FROM system.prepared_transactions`,
+		)
+		if err != nil {
+			if pgerror.GetPGCode(err) == pgcode.UndefinedTable {
+				// If the table does not exist, we're in a mixed-version cluster and the
+				// feature is not supported yet. Just return an empty result.
+				// TODO(nvanbenschoten): Remove this logic when mixed-version support
+				// with v24.2 is no longer necessary.
+				return nil
+			}
+			return err
+		}
+		for _, row := range rows {
+			// NOTE: we can't map a 128-bit CockroachDB transaction ID to a 32-bit
+			// Postgres xid, so we just return zero for each transaction's xid. This
+			// is acceptable, as the gid is the important part of pg_prepared_xacts.
+			transaction := zeroVal
+			globalID := row[0]
+			prepared := row[1]
+			owner := tree.NewDName(string(tree.MustBeDString(row[2])))
+			database := tree.NewDName(string(tree.MustBeDString(row[3])))
+			if err := addRow(
+				transaction, // transaction
+				globalID,    // gid
+				prepared,    // prepared
+				owner,       // owner
+				database,    // database
+			); err != nil {
+				return err
+			}
+		}
 		return nil
 	},
-	unimplemented: true,
 }
 
 // pgCatalogPreparedStatementsTable implements the pg_prepared_statements table.