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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.
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.

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