Re: computational model of transactions

Brian Selzer wrote:

> "J M Davitt" <jdavitt_at_aeneas.net> wrote in message 
> news:QVSAg.63281$Eh1.62802_at_tornado.ohiordc.rr.com...
> 

[snip]

>>>>>>The semantics of the update involve modification, not replacement
>>>>>
>>>>>You obviously see a difference between modification and replacement. I
>>>>>don't. So please explain.

[snip]

>>>I'm back. I agree that the updates need to be isolated, but I disagree
>>>with the idea that the entire transaction needs to be isolated or
>>>serialized. It is only necessary to obtain an exclusive lock on the
>>>affected row at the time that the update to the shared resource occurs,
>>>so it's possible to have several other intervening transactions commit
>>>between the time that the transaction starts and the time that the update
>>>starts. My point is that it is not necessary to isolate the entire
>>>transaction, only that portion from the start of the update until the
>>>commit.
>>
>>Are we to understand that "it's possible to have several other
>>intervening transactions commit between the time that the
>>transaction starts and the time that the update starts" means
>>that you believe that at "the time the update starts" the value
>>of whatever attribute is being changed isn't the same as it was
>>when the transaction started?
>>

> 
> Yes.  The nature of the update makes this possible.  An update that simply 
> decreases inventory by 5 need not know the state of the inventory at the 
> time that the transaction started. If you issue,

[snip]

It would appear that you view "modification" and "replacement" as two different sorts of updates. To the database engines that are providing concurrency and correctness, those are indistinguishable, AFAIK.

Also, your transactions seem like accounting system concepts rather than database concepts.

While, in accounting, it seems to be possible to simply dump all the debits and credits in a hopper and allow them to be processed in random order, there comes a time when activity must be serialized. The bookkeeper that's cross-footing a page isn't going to be very happy with the clerk who wants to change an entry that's been footed in one column but not another. Received on Sat Aug 05 2006 - 16:48:31 CEST