Re: computational model of transactions

Brian Selzer wrote:
> "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.
>>
>>Well, each of these specifies a value for a
>>column in a row in a table.  You think
>>the expression denoting the value makes these
>>different and that the system should be able
>>to detect the difference.  I'm still
>>wondering, "How?"
>>

>>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?
>>

Holy bat, Crapman! "As an aside" you've either abandoned the points you made earlier in this thread or have been using terminology inconsistently. When you started with

    I disagree with the idea that the entire transaction     needs to be isolated or serialized.

followed by

    it's possible to have several other intervening     transactions commit between the time that the     transaction starts and the time that the update     starts

and the hint at different types of UPDATE

    in this case intervening transactions cannot     have occurred, otherwise the values set by the     intervening transactions will be overwritten.     Hence with replacement, the row to be updated     must be reserved for exclusive use (at least for     writing) throughout the transaction.

which the system is supposed to detect

    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.

by examining both sides of the assignment, I thought you were trying to describe some computational model of transactions in which the system, based on the nature of the UPDATEs in the transaction, serialized parts of "replacement" transactions and other transactions that read "replaced" or "modified" data.

I was about to move on to the question, "If the system can detect that a 'replacement' UPDATE is in the mix, why not require it to optimize the workload and discard all the 'modify' UPDATEs?"

>>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?
>>

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