Re: why do you apply undo before redo?
Date: Sun, 18 Apr 2004 21:25:26 GMT
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
>>>"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
>>>>>I'm reading a generic database textbook and it states that when
> > databases >>>>I didn't want to type it. Its 'Database Systems Concepts'. The one used
>>>>>are recovering undo is applied before redo.
>>>>Any specific reason why you would like the title of the book to remain a
>>Good book. But I don't know many universities around here that use it.
>>Would you happen to have any statistics on that?
> >>>>Not quite sure how the undo can conflict. You roll forward and apply all
>>>>>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.
>>>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?\
> > 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.
- Jan Hidders