Zend_Session_SaveHandler_Db-don'；t删除旧会话

我使用db表来存储会话信息，但旧会话不会自动从表中删除。在引导程序类中，我有配置会话存储句柄的方法：

protected function _initSessionHandle(){
        //setup your DB connection like before
        //NOTE: this config is also passed to Zend_Db_Table so anything specific
        //to the table can be put in the config as well
        $oMultiDb = Zend_Registry::get('multidb');
        $oDbAdapter = $oMultiDb->getDb('db2');
        $aConfig = array(
            'name'          => 'session', //table name as per Zend_Db_Table
            'primary'       => array(
                'id',   //the sessionID given by PHP
            ),
            'modifiedColumn' => 'modified',     //time the session should expire
            'dataColumn'     => 'data', //serialized data
            'lifetimeColumn' => 'lifetime',     //end of life for a specific record
            'db'             => $oDbAdapter,
        );
        //Tell Zend_Session to use your Save Handler
        $oSessionSaveHandle = new Zend_Session_SaveHandler_DbTable($aConfig);
        //$oSessionSaveHandle->gc(0);
        Zend_Session::setSaveHandler($oSessionSaveHandle);
        Zend_Session::start();
    }

只有当我在代码$oSessionSaveHandle->gc(0);中设置时，旧会话才会被删除。为什么这不是自动发生的？当我们转到Zend_Session：：setSaveHandler方法声明时

session_set_save_handler(
        array(&$saveHandler, 'open'),
        array(&$saveHandler, 'close'),
        array(&$saveHandler, 'read'),
        array(&$saveHandler, 'write'),
        array(&$saveHandler, 'destroy'),
        array(&$saveHandler, 'gc')
        );

我们看到gc（）方法被传递给session_set_handle，但不起作用，为什么？