Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

CHAPTER 9 ■ REDO AND UNDO 299
Table 9-2. Time to ROLLBACK by Transaction Size
Rows Inserted Rollback Time Commit Time
(Seconds) (Seconds)
10 0.04 0.06
100 0.05 0.04
1,000 0.06 0.06
10,000 0.22 0.06
100,000 1.46 0.07
This is to be expected, as a ROLLBACK has to physically undo the work we’ve done. Similar
to a COMMIT, a series of operations must be performed. Before we even get to the ROLLBACK, the
database has already done a lot of work. To recap, the following would have happened:
• Undo segment records have been generated in the SGA.
• Modified data blocks have been generated in the SGA.
• A buffered redo log for the preceding two items has been generated in the SGA.
• Depending on the size of the preceding three items, and the amount of time spent,
some combination of the previous data may be flushed onto disk already.
• All locks have been acquired.
When we ROLLBACK,
• We undo all of the changes made. This is accomplished by reading the data back from
the undo segment, and in effect, reversing our operation and then marking the undo
entry as applied. If we inserted a row, a ROLLBACK will delete it. If we updated a row, a
rollback will reverse the update. If we deleted a row, a rollback will re-insert it again.
• All locks held by our session are released, and everyone who was enqueued waiting on
locks we held will be released.
A COMMIT, on the other hand, just flushes any remaining data in the redo log buffers. It
does very little work compared to a ROLLBACK. The point here is that you don’t want to roll back
unless you have to. It is expensive since you spend a lot of time doing the work, and you’ll also
spend a lot of time undoing the work. Don’t do work unless you’re sure you are going to want
to COMMIT it. This sounds like common sense—of course I wouldn’t do all of the work unless I
wanted to COMMIT it. Many times, however, I’ve seen a situation where a developer will use a
“real” table as a temporary table, fill it up with data, report on it, and then roll back to get rid
of the temporary data. In the next section, we’ll talk about true temporary tables and how to
avoid this issue.