Oracle PL/SQL之自定义函数的读一致性
当我们将自定义函数应用于sql语句中，而该sql又需要执行很长时间，并且在这段时间中恰好又有其他dml会更改该sql中的某些表时，这
读一致性即确保查询的结果与发起查询的时刻的数据一致，不管在此查询期间其他事务有没有更改数据。
首先假设我们有如下函数用来根据部门id求该部门的总薪资：
然后我们开个session（s1），，执行如下查询（q1）：
在q1执行的过程中，我们又开个新的session（s2），执行更新命令并提交：
接着回到s1，看q1的执行结果：
我们发现sum(sal)和sum2(deptno)的结果不一致。
当我们将自定义函数应用于sql语句中，而该sql又需要执行很长时间，并且在这段时间中恰好又有其他dml会更改该sql中的某些表时，这种不一致就会体现出来。
关于如何避免这种不一致，可以使用SET TRANSACTION READ ONLY，详细内容可参看我下面的这篇文章：
Oracle PL/SQL之SET TRANSACTION READ ONLY（事务隔离性）
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