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UNDO_RETENTION specifies (in seconds) the low threshold value of undo retention. 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.

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 the UNDO_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.

UNDO_RETENTION指定(在几秒钟内)低阈值解开保留。 系统保留解开在这个参量指定的至少时间和自动地调整解开保留阶段满足询问的解开要求。
这个参量设置应该占系统的所有反燃要求。 自动调整解开保留不为高吊球支持。 保留价值为高吊球专栏被设置到UNDO_RETENTION参量的价值。
如果潮流解开表空间有足够的空间， UNDO_RETENTION参量可能只荣幸。 如果一张活动事务处理要求解开空间，并且解开表空间没有可用空间，则系统开始重复利用未到期解开空间。 这次行动可能潜在地造成有些询问无法与“快照太老”消息。
解开的时间为Oracle数据库保留为潮流解开表空间可以通过询问V$UNDOSTAT动态性能视图的TUNED_UNDORETENTION专栏获得。