Undo segments and performance

Undo segments and performance

I have a job that truncates a huge table and inserts data into it. This job runs once a day. Searching for ways to optimize the insert, I found an 'append hint' that seems to bring me better results. I read that this hint does not create undo logs, and that's my question. I read all the time about redo logs but never heard of undo logs. What are they? I am running Oracle 8i and the subject table has nologging set.

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Undo segments (not logs) were called "rollback segments" prior to Oracle9i. Oracle writes the previous contents of database blocks to them when performing DML; this is what allow you to roll back DML statements. Since writes to undo segments are database writes (these segments are stored in a tablespace on disk like all other user data), changes to them are normally logged as redo. In the case of insert-append with a nologging hint, neither the user data changes nor the undo segment changes are logged in the redo stream.

This was first published in June 2005

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