sql: add missing obj_description case

Found with the following query:

```sql
COMMENT ON SCHEMA public IS 'hello';

SELECT obj_description(oid, 'pg_namespace')
FROM pg_namespace WHERE nspname = 'public';
```

Release note (sql change): The PostgreSQL compatibility
function `obj_description` now supports retrieving
comments on schemas.

pkg/sql/BUILD.bazel

@@ -536,6 +536,7 @@ go_test(
         "comment_on_constraint_test.go",
         "comment_on_database_test.go",
         "comment_on_index_test.go",
+        "comment_on_schema_test.go",
         "comment_on_table_test.go",
         "conn_executor_internal_test.go",
         "conn_executor_savepoints_test.go",

pkg/sql/comment_on_schema_test.go

@@ -0,0 +1,109 @@
+// Copyright 2022 The Cockroach Authors.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+package sql_test
+
+import (
+	"context"
+	gosql "database/sql"
+	"testing"
+
+	"github.com/cockroachdb/cockroach/pkg/sql/tests"
+	"github.com/cockroachdb/cockroach/pkg/testutils/serverutils"
+	"github.com/cockroachdb/cockroach/pkg/util/leaktest"
+	"github.com/cockroachdb/cockroach/pkg/util/log"
+	"github.com/cockroachdb/errors"
+)
+
+func TestCommentOnSchema(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+	defer log.Scope(t).Close(t)
+
+	params, _ := tests.CreateTestServerParams()
+	s, db, _ := serverutils.StartServer(t, params)
+	defer s.Stopper().Stop(context.Background())
+
+	if _, err := db.Exec(`
+		CREATE SCHEMA d;
+	`); err != nil {
+		t.Fatal(err)
+	}
+
+	testCases := []struct {
+		exec   string
+		query  string
+		expect gosql.NullString
+	}{
+		{
+			`COMMENT ON SCHEMA d IS 'foo'`,
+			`SELECT obj_description(oid, 'pg_namespace') FROM pg_namespace WHERE nspname = 'd'`,
+			gosql.NullString{String: `foo`, Valid: true},
+		},
+		{
+			`ALTER SCHEMA d RENAME TO d2`,
+			`SELECT obj_description(oid, 'pg_namespace') FROM pg_namespace WHERE nspname = 'd2'`,
+			gosql.NullString{String: `foo`, Valid: true},
+		},
+		{
+			`COMMENT ON SCHEMA d2 IS NULL`,
+			`SELECT obj_description(oid, 'pg_namespace') FROM pg_namespace WHERE nspname = 'd2'`,
+			gosql.NullString{Valid: false},
+		},
+	}
+
+	for _, tc := range testCases {
+		if _, err := db.Exec(tc.exec); err != nil {
+			t.Fatal(err)
+		}
+
+		row := db.QueryRow(tc.query)
+		var comment gosql.NullString
+		if err := row.Scan(&comment); err != nil {
+			t.Fatal(err)
+		}
+		if tc.expect != comment {
+			t.Fatalf("expected comment %v, got %v", tc.expect, comment)
+		}
+	}
+}
+
+func TestCommentOnSchemaWhenDrop(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+	defer log.Scope(t).Close(t)
+
+	params, _ := tests.CreateTestServerParams()
+	s, db, _ := serverutils.StartServer(t, params)
+	defer s.Stopper().Stop(context.Background())
+
+	if _, err := db.Exec(`
+		CREATE SCHEMA d;
+	`); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := db.Exec(`COMMENT ON SCHEMA d IS 'foo'`); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := db.Exec(`DROP SCHEMA d`); err != nil {
+		t.Fatal(err)
+	}
+
+	row := db.QueryRow(`SELECT comment FROM system.comments LIMIT 1`)
+	var comment string
+	err := row.Scan(&comment)
+	if !errors.Is(err, gosql.ErrNoRows) {
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		t.Fatal("dropped comment remain comment")
+	}
+}

pkg/sql/sem/builtins/pg_builtins.go

@@ -2169,6 +2169,8 @@ func getCatalogOidForComments(catalogName string) (id int, ok bool) {
 		return catconstants.PgCatalogDescriptionTableID, true
 	case "pg_constraint":
 		return catconstants.PgCatalogConstraintTableID, true
+	case "pg_namespace":
+		return catconstants.PgCatalogNamespaceTableID, true
 	default:
 		// We currently only support comments on pg_class objects
 		// (columns, tables) in this context.