Flash Back Corrections

The default value of RETENTION GUARANTEE is NO GUARANTEE. When this option is enabled, the database never overwrites unexpired undo data. By default, this option is disabled so that the database can overwrite the unexpired undo data in the undo tablespace.

The default value for the undo_retention parameter, which is set in seconds, is 900. The undo_retention parameter can be changed in the initialization parameter file to be used by the database startup process or at any time by using the ALTER SYSTEM statement.

In the initialization parameter file, put undo_retention = 1500 for 25 minutes, or issue the following SQL command:

ALTER SYSTEM SET UNDO_RETENTION = 1500;

This takes effect immediately, as long as there is free space in the current undo tablespace. The undo retention period for the current undo tablespace is obtained by querying the TUNED_UNDORETENTION column of the v$undostat dynamic performance view.

The RETENTION column is set to GUARANTEE when the tablespace has RETENTION GUARANTEE. Use this GUARANTEE clause with caution, as it may affect any existing or long running DML operations.