Re: why do you apply undo before redo?
Date: Sun, 18 Apr 2004 18:10:42 GMT
Message-ID: <Cwzgc.76768$nU5.5168180_at_phobos.telenet-ops.be>
Ryan wrote:
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
> news:Isugc.75959$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.
Good book. But I don't know many universities around here that use it.
Would you happen to have any statistics on that?
>>>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.
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?
Of course most concurrency control mechanisms that guarantee serializability would prevent that.
- Jan Hidders