You specify the minimum undo retention period (in seconds) by setting the UNDO_RETENTION initialization parameter. As described in "About the Undo Retention Period", the current undo retention period may be automatically tuned to be greater than UNDO_RETENTION, or, unless retention guarantee is enabled, less than UNDO_RETENTION if space in the undo tablespace is low.
To set the minimum undo retention period:
Do one of the following:
Set UNDO_RETENTION in the initialization parameter file.
UNDO_RETENTION = 1800
Change UNDO_RETENTION at any time using the ALTER SYSTEM statement:
ALTER SYSTEM SET UNDO_RETENTION = 2400;
The effect of an UNDO_RETENTION parameter change is immediate, but it can only be honored if the current undo tablespace has enough space.