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General Questions


tuned_undoretention in v$undostat


Dear all,  is the value from the column tuned_undoretention in the view v$undostat always the value from the parameter undo_retention?  Thanks a bunch, ...

I have different values there (most of the values are equal with my undo_retention but several different also there). Also you may want to check the docs.

No: http://download.oracle.com/docs/cd/E11882_01/server.112/e25494/undo002.htm#ADMIN12328 says:   Oracle Database automatically tunes the undo retention period based on how the undo tablespace is configured.      If the undo tablespace is configured with the AUTOEXTEND option, the database dynamically tunes the undo retention period to be somewhat longer than the longest-running active query on the system. However, this retention period may be insufficient to accommodate Oracle Flashback operations. Oracle Flashback operations resulting in snapshot too old errors are the indicator that you must intervene to ensure that sufficient undo data is retained to support these operations. To better accommodate Oracle Flashback features, you can either set the UNDO_RETENTION parameter to a value equal to the longest expected Oracle Flashback operation, or you can change the undo tablespace to fixed size.      If the undo tablespace is fixed size, the database dynamically tunes the undo retention period for the best possible retention for that tablespace size and the current system load. This best possible retention time is typically significantly greater than the duration of the longest-running active query.


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