session：用户对数据库的连接
A session is a specific connection of a user to an Oracle instance through a user process. For example, when a user starts SQL*Plus, the user must provide a valid username and password, and then a session is established for that user. A session lasts from the time the user connects until the time the user disconnects or exits the database application.
transaction：简单的说就是从开始或上一次commit起,到这次commit为止，当然有些命令是隐含commit的。
A transaction is a logical unit of work that comprises one or more SQL statements executed by a single user. According to the ANSI/ISO SQL standard, with which Oracle is compatible, a transaction begins with the user’s first executable SQL statement. A transaction ends when it is explicitly committed or rolled back by that user.
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