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

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