private function beginTransaction()
    {
        if (!$this->inTransaction) {
            if ('sqlite' === $this->driver) {
                $this->pdo->exec('BEGIN IMMEDIATE TRANSACTION');
            } else {
                if ('mysql' === $this->driver) {
                    $this->pdo->exec('SET TRANSACTION ISOLATION LEVEL READ COMMITTED');
                }
                $this->pdo->beginTransaction();
            }
            $this->inTransaction = true;
        }
    }