Re: computational model of transactions

"J M Davitt" <jdavitt_at_aeneas.net> wrote in message news:LnaBg.63487$Eh1.25115_at_tornado.ohiordc.rr.com...
> Brian Selzer wrote:

>> "J M Davitt" <jdavitt_at_aeneas.net> wrote in message 
>> news:AG6Bg.53697$u11.51832_at_tornado.ohiordc.rr.com...
>>
>>>Brian Selzer wrote:
>>>
>>>>"J M Davitt" <jdavitt_at_aeneas.net> wrote in message 
>>>>news:3d2Bg.44572$vl5.12370_at_tornado.ohiordc.rr.com...
>>>>
>>>>
>>>>>Brian Selzer wrote:
>>>>>
>>>>>
>>>>>>"J M Davitt" <jdavitt_at_aeneas.net> wrote in message 
>>>>>>news:QVSAg.63281$Eh1.62802_at_tornado.ohiordc.rr.com...
>>>>>>
>>>>>>
>>>>>>
>>>>>>>Brian Selzer wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>"Brian Selzer" <brian_at_selzer-software.com> wrote in message 
>>>>>>>>news:voHAg.4447$uo6.79_at_newssvr13.news.prodigy.com...
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>>"Erwin" <e.smout_at_myonline.be> wrote in message 
>>>>>>>>>news:1154689817.830401.130180_at_75g2000cwc.googlegroups.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.
>>>>>
>>>>
>>>>
>>>>Yes, I do.  Modification depends on the current state of the attribute; 
>>>>whereas replacement doesn't.
>>>>Database engines can provide concurrency and consistency, not 
>>>>correctness, so in a replacement, the assumption is that the new value 
>>>>is correct, and it's up to the application to correctly calculate the 
>>>>new value; whereas with modification, the new value is calculated by the 
>>>>database engine.  This means that for replacement it's also up to the 
>>>>application to request the correct level of concurrency, which can be 
>>>>more restrictive for replacement than for modification.
>>>
>>>Well, you're right about the consistent v. correct part, at
>>>least in the sense that the system has no way to determine
>>>whether or not what it's being asked to store is true in
>>>the real world.
>>>
>>>But you seem to have completely avoided my point that
>>>"replacement" and "modification" are the same thing for the
>>>database.  How do you think the system can tell the difference?
>>>
>>
>>
>> Here's an example of a replacement:
>>
>> UPDATE Inventory
>>     SET QOH = 35
>>     WHERE PartNo = '123'
>>         AND Location = 'ABC'
>>
>> Here's an example of a modification
>>
>> UPDATE Inventory
>>     SET QOH = QOH - 5
>>     WHERE PartNo = '123'
>>         AND Location = 'ABC'
>>
>> I think it's pretty clear which is which.  I think that the system should 
>> be able to detect the difference just as you can.
>

A compiler can tell the difference between x = 10 and x = x + 5, why can't a dbms?

> Exploring this a bit further: how many types
> of UPDATEs is the system supposed to detect?
> I mean, if we start by saying the number's
> greater than one, how do we know when they
> are all covered? Are you sure that
> modification and replacement are all there
> are?
>

The system should be able to detect whether or not the new value depends on the previous value. The first UPDATE statement above does not, the second does.

As an aside, it is not really necessary that the system detect this: but the developer must, because in a concurrent environment the difference in the semantics of replacement and modification has ramifications that can affect the appropriate choice of transaction isolation level.

> While we're talking manipulations: what about
> INSERT and DELETE? Are there variants of
> those, too? Are those supposed to be handled
> differently in transaction context?
>

I haven't given this much attention, but at first glance, no, I don't think so.

>>>>>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.
>>>>
>>>>
>>