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_at_phobos.telenet-ops.be>
>
> 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?
Date: Sun, 18 Apr 2004 22:53:27 GMT
Message-ID: <HFDgc.77172$4W5.5180553_at_phobos.telenet-ops.be>
Ryan wrote:
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
> news:anCgc.77108$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?
- Jan Hidders