Re: computational model of transactions


"J M Davitt" <jdavitt@xxxxxxxxxx> wrote in message
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Brian Selzer wrote:
"J M Davitt" <jdavitt@xxxxxxxxxx> wrote in message
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Brian Selzer wrote:

"J M Davitt" <jdavitt@xxxxxxxxxx> wrote in message
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Brian Selzer wrote:


"Brian Selzer" <brian@xxxxxxxxxxxxxxxxxxx> wrote in message
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"Erwin" <e.smout@xxxxxxxxxxx> wrote in message
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[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.

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.



.

Relevant Pages

  • Re: computational model of transactions
    ... replacement. ... disagree with the idea that the entire transaction needs to be ... the inventory at the time that the transaction started. ...
    (comp.databases.theory)
  • Re: computational model of transactions
    ... disagree with the idea that the entire transaction needs to be ... whereas replacement doesn't. ... Database engines can provide concurrency and consistency, ...
    (comp.databases.theory)
  • Re: computational model of transactions
    ... 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. ... 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. ...
    (comp.databases.theory)
  • Re: computational model of transactions
    ... replacement. ... transaction starts and the time that the update starts. ... Database engines can provide concurrency and consistency, ...
    (comp.databases.theory)
  • Re: computational model of transactions
    ... replacement. ... transaction starts and the time that the update starts. ... UPDATE Inventory ...
    (comp.databases.theory)