public function findExpiredIdentifiers(): array
    {
        $queryBuilder = $this->createQuery();
        $rows = $queryBuilder->select('identifier')
            ->from($this->getTableName())
            ->where($queryBuilder->expr()->lt(
                'expires',
                $queryBuilder->createNamedParameter((new DateTimeService())->getCurrentTime(), \PDO::PARAM_INT)
            ))
            ->groupBy('identifier')
            ->execute()
            ->fetchAll();

        $cacheIdentifiers = [];
        foreach ($rows as $row) {
            $cacheIdentifiers[] = $row['identifier'];
        }

        return $cacheIdentifiers;
    }