public function clean()
    {
        $delay = self::OUT_OF_SYNC_TIME_LIMIT;
        $query = $this->pdo->prepare(<<<SQL
DELETE FROM robotstxt__delay0
WHERE delayUntil < ((UNIX_TIMESTAMP() - :delay) * 1000000);
SQL
        );
        $query->bindValue('delay', $delay, \PDO::PARAM_INT);
        return $query->execute();
    }