undo表空间只做一件事：保存旧值。在AUM下，段是自动生成，区是自动分配与回收，且是不连续的。oracle自动使用undo段，在某种意义
undo表空间只做一件事：保存旧值。在AUM下，段是自动生成，区是自动分配与回收，且是不连续的。Oracle自动使用undo段，，在某种意义上，我们只要关注undo表空间的大小就可以了。
查询当前哪个undo表空间被激活：
查询undo表空间里面有多少个数据文件：
查询undo表空间里面在线有多少个段：
查询段有多少个区和块：
查询区有多少个块：
从这里我们也可以看出，undo段中的区的分配是不连续的。
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