protected function createConnection() {
        $config = $this->config;
        $db = new \mysqli;
        $db->connect($config['host'], $config['user'], $config['password'], $config['database'], $config['port']);
        if (!empty($config['charset'])) {
            $db->set_charset($config['charset']);
        }
        $db_sock = swoole_get_mysqli_sock($db);
        swoole_event_add($db_sock, array($this, 'onSQLReady'));
        $this->idle_pool[$db_sock] = array(
            'object' => $db,
            'socket' => $db_sock,
        );
        $this->connection_num++;
    }