public function findPageIdsOfExistingMountPagesByMountIdentifier(string $identifier) : array
    {
        $queryBuilder = $this->getQueryBuilder();
        $resultSet = $queryBuilder
            ->select('item_uid')
            ->add('select', $queryBuilder->expr()->count('*', 'queueItemCount'), true)
            ->from($this->table)
            ->where(
                $queryBuilder->expr()->eq('item_type', $queryBuilder->createNamedParameter('pages')),
                $queryBuilder->expr()->eq('pages_mountidentifier', $queryBuilder->createNamedParameter($identifier))
            )
            ->groupBy('item_uid')
            ->execute();

        $mountedPagesIdsWithQueueItems = [];
        while ($record = $resultSet->fetch()) {
            if ($record['queueItemCount'] > 0) {
                $mountedPagesIdsWithQueueItems[] = $record['item_uid'];
            }
        }

        return $mountedPagesIdsWithQueueItems;
    }