public function transactionStart($transactionMode = false, $sessionCharacteristics = false)
    {
        if ($this->transactionNesting > 0) {
            $this->transactionSavepoint('NESTEDTRANSACTION' . $this->transactionNesting);
        } elseif ($this->connector instanceof SQLServerConnector) {
            $this->connector->transactionStart();
        } else {
            $this->query('BEGIN TRANSACTION');
        }
        ++$this->transactionNesting;
    }