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Oracle® Database Reference 10g Release 2 (10.2) Part Number B14237-02 |
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# UNDO_RETENTION

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 ignores `UNDO_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 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.

See Also: Oracle Database SQL Reference for more information about creating undo tablespaces Oracle Database Administrator's Guide for more information about managing undo data