过程调用可以是嵌套的或递归的，但是保存点的范围被限制为定义它的存储过程的执行。
Procedure calls can be nested or recursive, but the scope of a savepoint is restricted to the execution of the stored procedure in which it is defined.
调用自身的函数就是递归的；执行这种函数的过程就叫递归运算。
A function that calls itself is recursive; the process of executing it is called recursion.
在我们的示例流程中，该递归过程是无限的；subprocess 3总会再次调用Main process。
For our example process, the recursion is infinite; Subprocess 3 always invokes Main process again.
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