private static function _checkSequence()
    {
        if (Driver::$primary_key_behaviour!=Driver::PRIMARY_KEY_SEQUENCE) {
            return null;
        }

        if (self::_sequenceExists()) {
            return;
        }

        switch (Driver::$name) {
        case "oracle":
            self::_createOracleSequence();
            break;
        case "postgresql":
            self::_createPostgresqlSequence();
            break;
        }
    }