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Re: why do you apply undo before redo?

From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Sun, 18 Apr 2004 22:53:27 GMT
Message-ID: <HFDgc.77172$4W5.5180553@phobos.telenet-ops.be>


Ryan wrote:
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
> news:anCgc.77108$TZ5.5226557_at_phobos.telenet-ops.be...

>>Ryan wrote:
>>>"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
>>>news:Cwzgc.76768$nU5.5168180_at_phobos.telenet-ops.be...
>>>>Suppose that in the REDO list there is a transaction with an operation
>>>>that changed a field from "yes" to "no", and in the UNDO list there is a
>>>>transaction with also an operation that changed the same field also from
>>>>"yes" to "no". Then what is the result of the field after REDO - UNDO
>>>>and what after UNDO - REDO?
>>>
>>>I do not see why this matters. It does not matter what the state of the
>>>tuple is during recovery, only when it is complete.
>>
>>That is what I'm talking about. What is the result of the field after
>>recovery has completed in the two cases? The first case being first REDO
>>and then UNDO and the second being the reverse.

>
> undo only takes place for uncomitted transactions. undo for committed
> transactions are ignored. are you telling me the author is assuming that ALL
> undo is applied even for committed transactions?

No, I'm not. In the example the transaction in the UNDO list did not yet commit, and the transaction in the REDO list did. Does the book not explain what the REDO and the UNDO list are and how they are constructed?

Received on Sun Apr 18 2004 - 17:53:27 CDT

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