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最新推荐文章于 2024-07-03 11:40:42 发布

原创最新推荐文章于 2024-07-03 11:40:42 发布 · 671 阅读

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本文详细解析了数据库参数UNDO_RETENTION的含义、作用范围、默认值、修改方式及适用场景，包括其对撤销数据保留时间的影响，以及如何在不同大小的undo表空间下进行自动调整。

7: The UNDO_RETENTION parameter in yourdatabase is set to 1000 and undo retention is not guaranteed.Which statementregarding retention of undo data is correct?

A. Undo data becomes obsolete after 1,000 seconds.

B. Undo data gets refreshed after every 1,000 seconds.

C. Undo data will be stored permanently after 1,000 seconds.

D. Committed undo data would be retained for 1,000 seconds if free undo space is available.

E. Undo data will be retained in the UNDO tablespace for 1,000 seconds,then it gets moved to the TEMPORARY tablespace to provide read consistency.

Property	Description
Parameter type	Integer
Default value	`900`
Modifiable	`ALTER SYSTEM`
Range of values	`0` to 2³² - 1 (max value represented by 32 bits)
Real Application Clusters	Oracle recommends that multiple instances have the same value.

UNDO_RETENTION specifies (in seconds) the low threshold value of undo retention. For AUTOEXTEND undo tablespaces, the system retains undo for at least the time specified in this parameter, and automatically tunes the undo retention period to satisfy the undo requirements of the queries. For fixed- size undo tablespaces, the system automatically tunes for the maximum possible undo retention period, based on undo tablespace size and usage history, and ignoresUNDO_RETENTION unless retention guarantee is enabled.

The setting of this parameter should account for any flashback requirements of the system. Automatic tuning of undo retention is not supported for LOBs. The RETENTION value for LOB columns is set to the value of theUNDO_RETENTION parameter.

The UNDO_RETENTION parameter can only be honored if the current undo tablespace has enough space. If an active transaction requires undo space and the undo tablespace does not have available space, then the system starts reusing unexpired undo space. This action can potentially cause some queries to fail with a "snapshot too old" message.

The amount of time for which undo is retained for the Oracle Database for the current undo tablespace can be obtained by querying the TUNED_UNDORETENTION column of the V$UNDOSTAT dynamic performance view.