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undo segment [message #90488] Tue, 02 December 2003 18:40 Go to next message
Messages: 1
Registered: December 2003
Junior Member
undo segment is used to save the uncommit file....
i wanna know why the file must be save? for what?
thank you very much
Re: undo segment [message #90493 is a reply to message #90488] Thu, 04 December 2003 17:19 Go to previous message
William Robertson
Messages: 1643
Registered: August 2003
Location: London, UK
Senior Member
So you can roll back, either explicitly (you enter "ROLLBACK;") or something fails partway through.

Undo information is also used for read consistency, which is how Oracle is able to present you with a set of query results in a database where many other sessions are applying inserts/updates/deletes at the same time. You will see the data as it existed as at the point in time when you issued the query, and Oracle uses undo segments as part of this.
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