n.保存点
Review savepoint and mark options for transactions. 检查事务的保存点和标记选项。
Roll back to the given savepoint. 回滚到给定的回滚点。
Explicitly release the given savepoint. 释放一个给定的回滚点。
If a 547 error occurs, it rolls back to the savepoint and returns the number of items on hand to the calling process. 如果出现547错误,它将回滚到该保存点,并将当前手边有的项目数返回给调用进程。
This is consistent with the principle that ROLLBACK cancels all effects of the commands since the savepoint. 这点和 ROLLBACK 取消所有命令自 savepoint （保存点）以来的影响的原则是一致的。
