1.356 UNDO_RETENTION
UNDO_RETENTION
specifies (in seconds) the low threshold value of undo retention.
Property | Description |
---|---|
Parameter type |
Integer |
Default value |
|
Modifiable |
|
Modifiable in a PDB |
Yes |
Range of values |
|
Basic |
No |
Oracle RAC |
Oracle recommends that multiple instances have the same value. |
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.
You can modify this parameter in a PDB only if the PDB is in local undo mode; you cannot modify this parameter if the PDB is in shared undo mode.
See Also:
-
Oracle Database SQL Language Reference for information about creating undo tablespaces
-
Oracle Database Administrator’s Guide for information about managing undo data