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Introduction to Automatic Undo Management (29)

原创 IT综合 作者:tsinglee 时间:2007-09-14 16:00:32 0 删除 编辑

Oracle maintains information to nullify changes made to the database. Such
information consists of records of the actions of transactions, collectively known as
undo. Oracle uses the undo to do the following:
■ Rollback an active transaction
■ Recover a terminated transaction
■ Provide read consistency
■ Recovery from logical corruptions

Automatic undo management eliminates the complexities of managing rollback
segment space and lets you exert control over how long undo is retained before
being overwritten. Oracle strongly recommends that you use undo tablespaces to manage
undo rather than rollback segments. The system automatically tunes the period for
which undo is retained in the undo tablespace to satisfy queries that require undo
information. If the current undo tablespace has enough space, then you can set the
UNDO_RETENTION parameter to a low threshold value so that the system retains the
undo for at least the time specified in the parameter.

Use the V$UNDOSTAT view to monitor and configure your database system to achieve
efficient use of undo space. V$UNDOSTAT shows various undo and transaction
statistics, such as the amount of undo space consumed in the instance.

undo的作用:
1. 回滚活动的事务
2. 恢复终止的事务
3. 提供一致性读
4. Recovery from logical corruptions

Automatic Undo Management(AUM)
1. 减少管理回滚段的复杂性
2. 主要关注保留多久的回滚性息不被覆盖

主要视图:v$undostat

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