The OPTIMAL storage parameter in the rollback segment

Question

Hi,
in metalink note Subject: ORA-01555 "Snapshot too old" in Very Large Databases (if using Rollback Segments)
Doc ID: 45895.1
I see :
Solution 1d:
    Don't use the OPTIMAL storage parameter in the rollback segment. but how not to use the OPTIMAL storage parameter in the rollback segment ?
Thank you.

Answer 1

If you are using undo_management=AUTO (in 9i or higher) then there is no "OPTIMAL" setting.
"OPTIMAL" is when using Manual Undo Management with Rollback Segments created by the DBA.
If you are using Manual Undo Management, check your Rollback Segments. The Optimal size would be visible in V$ROLLSTAT.
select a.segment_name a,  b.xacts b, b.waits c, b.shrinks e, b.wraps f,
      b.extends g, b.rssize/1024/1024 h, b.optsize/1024/1024 i,
      b.hwmsize/1024/1024 j, b.aveactive/1024/1024 k , b.status l
-- from v$rollname a, v$rollstat b
from dba_rollback_segs a, v$rollstat b
where a.segment_id = b.usn(+)
and b.status  in ('ONLINE', 'PENDING OFFLINE','FULL')
order by a.segment_name
/To unset the Optimal setting you can run
alter rollback segment SEGMENT_NAME storage (optimal NULL);Note that if you unset OPTIMAL, then your Rollback Segments will remain at very large sizes if and when they grow running large transactions ("OPTIMAL" is the pre-9i method for Oracle to automatically shrink Rollback Segments). You can manually SHRINK or DROP and CREATE Rollback Segments then.