protected function _list_tables($prefix_limit = FALSE)
    {
        $sql = 'SELECT "RDB$RELATION_NAME" FROM "RDB$RELATIONS" WHERE "RDB$RELATION_NAME" NOT LIKE \'RDB$%\' AND "RDB$RELATION_NAME" NOT LIKE \'MON$%\'';

        if ($prefix_limit === TRUE && $this->dbprefix !== '')