Re: computational model of transactions

"Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message news:2LlBg.35719$pu3.465028_at_ursa-nb00s0.nbnet.nb.ca...
>J M Davitt wrote:
>
>> Brian Selzer wrote:
>>
>>> "J M Davitt" <jdavitt_at_aeneas.net> wrote in message
>>> news:LnaBg.63487$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?"
>>>
>>> A compiler can tell the difference between x = 10 and x = x + 5, why
>>> can't a dbms?
>
> A compiler can but would it? How many language grammars have you seen that
> had the following assignment productions:
>
> <varname> = <literal>
> <varname> = <varname> + <literal>
> ...
>
> instead of
>
> <varname> = <expression>
>

And <expression> is defined how? Doesn't it include <varname> in its definition?

>
>>>> 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.
>
> What about the following?
>
> UPDATE Inventory
> SET QOH = (
> SELECT MIN( i2.QOH - 5 )
> FROM Inventory i2
> WHERE PartNo = '123'
> AND Location = 'ABC'
> )
> WHERE PartNo = '123'
> AND Location = 'ABC'
>
>
>>> 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.
>>
>> Holy bat, Crapman!
>
> He is full of it, isn't he?

And you're not?

>
>
> "As an aside" you've either abandoned
>> the points you made earlier in this thread or have been
>> using terminology inconsistently.
>
> Or he has neither a clue nor a place to put one. He has already
> demonstrated that he is an idiot who speaks more to hear his own voice
> than for any other reason.

Maybe you should take the log out of your own eye. Received on Sun Aug 06 2006 - 18:00:08 CEST