Do Frequent Commits Fix ORA-01555?

I answered a question on the OTN forum this morning that I figured is pertinent enough to post on my blog…it’s a very common question among DBAs and developers, and I think there are plenty of myths surrounding it as well.

Question: I am updating 1 million rows in Oracle 10g platform, normally when I do it in oracle 9i I run it as batch process and commit after each batch. Obviously to avoid/control undo generation. But in Oracle 10g I am told undo management is automatic and I do not need run the update as batch process.

Is this right please throw some light on this new feature - automatic undo management

Answer: Automatic undo management was available in 9i as well, and my guess is you were probably using it there. However, I’ll assume for the sake of this writing that you were using manual undo management in 9i and are now on automatic.

Automatic undo management depends upon UNDO_RETENTION, a parameter that defines how long Oracle should try to keep committed transactions in UNDO. However, this parameter is only a suggestion. You must also have an UNDO tablespace that’s large enough to handle the amount of UNDO you will be generating/holding, or you will get ORA-01555: Snapshot too old, rollback segment too small errors.

You can use the UNDO advisor to find out how large this tablespace should be given a desired UNDO retention, or look online for some scripts…just google for: oracle undo size

Oracle 10g also gives you the ability to guarantee undo. This means that instead of throwing an error on SELECT statements, it guarantees your UNDO retention for consistent reads and instead errors your DML that would cause UNDO to be overwritten.

Now, for your original question…yes, it’s easier for the DBA to minimize the issues of UNDO when using automatic undo management. If you set the UNDO_RETENTION high enough with a properly sized undo tablespace you shouldn’t have as many issues with UNDO. How often you commit should have nothing to do with it, as long as your DBA has properly set UNDO_RETENTION and has an optimally sized UNDO tablespace. Committing more often will only result in your script taking longer, more LGWR/DBWR issues, and the “where was I” problem if there is an error (if it errors, where did it stop?).

Lastly (and true even for manual undo management), if you commit more frequently, you make it more possible for ORA-01555 errors to occur. Because your work will be scattered among more undo segments, you increase the chance that a single one may be overwritten if necessary, thus causing an ORA-01555 error for those that require it for read consistency.

It all boils down to the size of the undo tablespace and the undo retention, in the end…just as manual management boiled down to the size, amount, and usage of rollback segments. Committing frequently is a peroxide band-aid: it covers up the problem, tries to clean it, but in the end it just hurts and causes problems for otherwise healthy processes.