protected function getLastSearchesResultSet($limit) : array
    {
        $queryBuilder = $this->getQueryBuilder();
        return $queryBuilder
            ->select('keywords')
            ->addSelectLiteral(
                $queryBuilder->expr()->max('tstamp','maxtstamp')
            )
            ->from($this->table)
            // There is no support for DISTINCT, a ->groupBy() has to be used instead.
            ->groupBy('keywords')
            ->orderBy('maxtstamp', 'DESC')
            ->setMaxResults($limit)->execute()->fetchAll();
    }