一旦删除，文件不能撤消删除，除非可能是由第三方恢复实用工具。
Once removed, the files cannot be undeleted, except possibly by a third-party recovery utility.
注对于添加了空间列的源表，根本不需要这个触发器；一旦删除了一行，它就消失了。
NOTE in the case of a single, integrated table, this trigger is not needed at all; once a row is deleted, that's it.
一旦删除它，剩下的顶点中必然有一个没有后继，所以它成为下一个拓扑序列中的最后一个，依此类推。
As soon as it's removed, one of the remaining vertices must have no successors, so it will be the next-to-last one in the ordering, and so on.
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