private function _registerTag($id, $tag) {
        $res = $this->_query("DELETE FROM TAG WHERE name='$tag' AND id='$id'");
        $res = $this->_query("INSERT INTO tag (name, id) VALUES ('$tag', '$id')");
        if (!$res) {
            $this->_log("Zend_Cache_Backend_Sqlite::_registerTag() : impossible to register tag=$tag on id=$id");
            return false;
        }
        return true;
    }