Unable to shrink undo tablespace... Help!

Question

Hi,
I have problems to shrink the system undo tablespace, which has grown up to 14 GB.
I use 9.2. Table space owner is 'SYSTEM', undo_management = AUTO.
I tried to shrink the greatest rollback segments by the commands
ALTER SESSION SET UNDO_SUPPRESS_ERRORS = TRUE;
ALTER ROLLBACK SEGMENT "_SYSSMU6$" SHRINK TO 20 M;
Oracle confirmed these commands, but nothing happened.
What am I doing wrong?
Hermann Mueller

Answer 1

You have seen the discussion about the undo segments, on the temporary tablespaces, you should be aware that the sort segment of a given temporary tablespace is created at the time the first sort operation takes place. The sort segment continues to grow by means of extent allocation until the segment size has reached the total storage demands of all of the active sorts running on the instance. Oracle will keep on allocating temporary space on demand unless the physical limit states otherwise.
Temporary segments are produced each time a sort operation (explicit -order by- or implicit -aggregation, reindexing-) requires to sort a set that cannot fit into memory. So if you detect excesive sort usage you should aim your monitors towards the sort operations (reports, reindexing, max, min, aggregations, order by ...). If your system has a DDS behaviour, this kind of operations are frequent as a massive sorting has to be peformed against millions of rows.
A Temporary tablespace will almost always appear to be near 100% full, that's because once oracle has allocated temporary space it doesn't release it back to the free space, it keeps it allocated even when the sort operation has finished. Criteria behind this fact is similar to the one oracle used to have when the rollback segments were in use, Oracle only allocated space and the dba should perform manual actions to release space, and the criteria is performance. Once it has allocated space this big, there are possibilities that the same circumstances that raised the temporary usage high water mark to this level are repeated, so if oracle keeps the mamimum allocated space, it won't have to allocate the same storage once more.
Main concern with temporary tablespace growth is not free space itself, but the reasons why this amount of space was allocated, so I suggest you to track the sql statements with sort operations. If you are certain the circumstances that motivated this amount of temporary resources to be allocated won't be repeated again, then you could think of resizing down your temporary tablespace. I suggest you to create a new temporary tablespace with the desired size, and alter the default tempoary tablespace to point to this newly crated temporary tablespace, and finally get rid of the original temporary TS.
~ Madrid