builtins: implement _pg_index_position as UDF

Release note: None

pkg/sql/sem/builtins/pg_builtins.go

@@ -2056,43 +2056,20 @@ SELECT description
 	// _pg_index_position return the column's position in the index
 	// (or NULL if not there).
 	//
-	// NOTE: this could be defined as a user-defined function, like
-	// it is in Postgres:
+	// NOTE: this is defined as a UDF, same as in Postgres:
 	// https://github.com/postgres/postgres/blob/master/src/backend/catalog/information_schema.sql
-	//
-	//  CREATE FUNCTION _pg_index_position(oid, smallint) RETURNS int
-	//      LANGUAGE sql STRICT STABLE
-	//  BEGIN ATOMIC
-	//  SELECT (ss.a).n FROM
-	//    (SELECT information_schema._pg_expandarray(indkey) AS a
-	//     FROM pg_catalog.pg_index WHERE indexrelid = $1) ss
-	//    WHERE (ss.a).x = $2;
-	//  END;
-	//
 	"information_schema._pg_index_position": makeBuiltin(defProps(),
 		tree.Overload{
+			IsUDF: true,
 			Types: tree.ParamTypes{
 				{Name: "oid", Typ: types.Oid},
 				{Name: "col", Typ: types.Int2},
 			},
 			ReturnType: tree.FixedReturnType(types.Int),
-			Fn: func(ctx context.Context, evalCtx *eval.Context, args tree.Datums) (tree.Datum, error) {
-				r, err := evalCtx.Planner.QueryRowEx(
-					ctx, "information_schema._pg_index_position",
-					sessiondata.NoSessionDataOverride,
-					`SELECT (ss.a).n FROM
-					  (SELECT information_schema._pg_expandarray(indkey) AS a
-					   FROM pg_catalog.pg_index WHERE indexrelid = $1) ss
-            WHERE (ss.a).x = $2`,
-					args[0], args[1])
-				if err != nil {
-					return nil, err
-				}
-				if len(r) == 0 {
-					return tree.DNull, nil
-				}
-				return r[0], nil
-			},
+			Body: `SELECT (ss.a).n FROM
+		         (SELECT information_schema._pg_expandarray(indkey) AS a
+			        FROM pg_catalog.pg_index WHERE indexrelid = $1) ss
+			       WHERE (ss.a).x = $2`,
 			Info:       notUsableInfo,
 			Volatility: volatility.Stable,
 		},