public function gc_db()
    {
        // just clean all old sessions when this GC is called
        $this->db->query("DELETE FROM " . $this->db->table_name('session')
            . " WHERE changed < " . $this->db->now(-$this->gc_enabled));

        $this->log("Session GC (DB): remove records < "
            . date('Y-m-d H:i:s', time() - $this->gc_enabled)
            . '; rows = ' . intval($this->db->affected_rows()));
    }