Re: Mistake in statement example

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От David G. Johnston
Тема Re: Mistake in statement example
Дата
Msg-id CAKFQuwadh0Q9WgQjRL=Uq8Fdm8pi6gfVpjX=ZWHNNTVkRpnvkg@mail.gmail.com
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Ответ на Re: Mistake in statement example  (Tom Lane <tgl@sss.pgh.pa.us>)
Ответы Re: Mistake in statement example  (Bruce Momjian <bruce@momjian.us>)
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On Wed, Mar 1, 2023 at 9:34 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
PG Doc comments form <noreply@postgresql.org> writes:
> I believe there is a mistake in an example on
> https://www.postgresql.org/docs/current/transaction-iso.html section
> 13.2.1:
> BEGIN;
> UPDATE accounts SET balance = balance + 100.00 WHERE acctnum = 12345;
> UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 7534;
> COMMIT;

> The acctnum is expected to be 12345 in both cases.

No, I think that's intentional: the example depicts transferring
$100 from account 7534 to account 12345.


That may be, but the descriptive text and point of the example (which isn't atomicity, but concurrency) doesn't even require the second update command to be present.  What the example could use is a more traditional two-session depiction of the commands instead of having a single transaction and letting the user envision the correct concurrency.

Something like:

S1: SELECT balance FROM accounts WHERE acctnum = 12345; //100
S1: BEGIN;
S2: BEGIN;
S1: UPDATE accounts SET balance = balance + 100.00 WHERE acctnum = 12345; //200
S2: UPDATE accounts SET balance = balance + 100.00 WHERE acctnum = 12345; //WAITING ON S1
S1: COMMIT;
S2: UPDATED; balance = 300
S2: COMMIT;

Though maybe "balance" isn't a good example domain, the incrementing example used just after this one seems more appropriate along with the added benefit of consistency.

David J.

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