protected function checkConnection($reconnect = false)
        {
            if(empty($this->_lnk)) {
                $this->log('CheckConnection: link is not valid');
                if($reconnect) {
                    $this->log('CheckConnection: try to reconnect');
                    $this->_lnk = @odbc_connect('Driver=' . VERTICA_DRIVER . ';Servername=' . VERTICA_SERVER . ';Database='.VERTICA_DATABASE, VERTICA_USER_ETL, VERTICA_PASS_ETL);
                    if(!$this->_lnk) {
                        $this->log('CheckConnection: reconnect failed');
                        throw new Exception('Connection failed or gone away and can\'t reconnect - '.odbc_errormsg($this->_link), self::CONNECTION_FAILED);
                    }
                    $this->log('CheckConnection: reconnected!');
                    return;
                }
                throw new Exception('Connection failed or gone away - ' . odbc_errormsg($this->_link), self::CONNECTION_FAILED);
            }
        }