static function gc()
    {
        $rcube = rcube::get_instance();
        $db    = $rcube->get_dbh();
        $now   = $db->now();

        $db->query("DELETE FROM " . $db->table_name('cache_messages', true)
              ." WHERE `expires` < $now");

        $db->query("DELETE FROM " . $db->table_name('cache_index', true)
              ." WHERE `expires` < $now");

        $db->query("DELETE FROM ".$db->table_name('cache_thread', true)
              ." WHERE `expires` < $now");
    }