protected function getAppliedChangeNumbers()
    {
        if (count($this->appliedChangeNumbers) == 0) {
            $this->log('Getting applied changed numbers from DB: ' . $this->url);
            $appliedChangeNumbers = [];
            $dbh = new PDO($this->url, $this->userid, $this->password);
            $dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            $this->dbmsSyntax->applyAttributes($dbh);
            $sql = "SELECT *
                    FROM " . DbDeployTask::$TABLE_NAME . "
                    WHERE delta_set = '$this->deltaSet'
                    ORDER BY change_number";
            foreach ($dbh->query($sql) as $change) {
                $appliedChangeNumbers[] = $change['change_number'];
            }
            $this->appliedChangeNumbers = $appliedChangeNumbers;
        }

        return $this->appliedChangeNumbers;
    }