In automatic undo management mode, the system controls exclusively the
assignment of transactions to undo segments, and controls space allocation for undo segments. An ill-behaved transaction can potentially consume much of the undo space, thus paralyzing the entire system. The Resource Manager directive UNDO_POOL is a more explicit way to control large transactions. This lets database administrators group
users into consumer groups, with each group assigned a maximum undo space limit. When the total undo space consumed by a group exceeds the limit, its users cannot make further updates until undo space is freed up by other member transactions ending.
The default value of UNDO_POOL is UNLIMITED, where users are allowed to consume as much undo space as the undo tablespace has. Database administrators can limit a particular user by using the UNDO_POOL directive.
1. 可以将某些用户编为一组 ， 并且给该组使用回滚表空间的 上限 。在该组其他用户释放空间之前 ， 组内用户不能进行
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