一旦用户从数据库服务器断开连接，会话将消失。
Once the user disconnects from the database server, the session disappears.
POP3 允许用户连接、获取和发送邮件，然后断开连接。
POP3 allows users to connect, retrieve and send mail, and then disconnect.
但是用户必须理解，断开连接的对象是保存在内存中。
But the user must understand that disconnected objects are kept in memory.
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