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Re: why do you apply undo before redo?

From: Ryan <rgaffuri_at_cox.net>
Date: Sun, 18 Apr 2004 13:21:19 -0400
Message-ID: <fQygc.5050$uF3.2587@lakeread04>

"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message news:Isugc.75959$VI4.5039222_at_phobos.telenet-ops.be...
> Ryan wrote:
> > I'm reading a generic database textbook and it states that when
databases
> > are recovering undo is applied before redo.
>
> Any specific reason why you would like the title of the book to remain a
> secret? :-)

I didn't want to type it. Its 'Database Systems Concepts'. The one used everywhere.

>
> > It doesn't say why. Does anyone know?
>
> Because some of the operations of the transactions of the UNDO list
> might conflict with those in the REDO list. So if you did the undo-phase
> last you might be erasing the result of some operations that you first
> did in the redo-phase.

don't quite get this. Oracle applies redo, then rollsback all open transactions? This is why that statement intrigued me since one database system does it the opposite way.

Not quite sure how the undo can conflict. You roll forward and apply all change vectors, then rollback and remove what was never committed.
>
> -- Jan Hidders
Received on Sun Apr 18 2004 - 12:21:19 CDT

Original text of this message

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