Message-ID: <5f459c76d3decb4e2e9e8b5a2a29daed0e99e0ce.camel@cybertec.at>
Subject: Re: Locking
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Anthony Berglas <anthony@berglas.org>
Cc: pgsql-docs@lists.postgresql.org
Date: Sat, 16 Mar 2024 14:48:32 +0100
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On Sat, 2024-03-16 at 10:52 +1000, Anthony Berglas wrote:
> The code is wrong by any normal definition.=C2=A0 We lose one of the upda=
tes.
> It is a very common error.=C2=A0 It=C2=A0is a very common way to write co=
de,
> especially if using an ORM, which is very common.
>=20
> [example of a lost update]
>
> The problem is that this fundamental problem is lost in all the escoteric
> details of locking.
>=20
> Sure, a database expert that carefully studies the docs might figure it
> out if they did not already know it.=C2=A0 But the other 99.9% of users w=
ill
> just consider Postgresql to be buggy because balances do not add up.
>=20
> So I think something in the docs is very much necessary.

Yes, the "lost update" is a common and well-known transaction anomaly,
and every developer should know about it.

What you are looking for is a tutorial about database transactions.
There are fundamental differences between a tutorial and documentation.
A tutorial is an example-based introduction aimed at beginners, while
the documentation describes the behavior in greater detail, aiming for
rigorourness and completeness.

Now there is a tutorial inside the PostgreSQL documentation, and it even
has a chapter about transactions:
https://www.postgresql.org/docs/current/tutorial-transactions.html
It even talks some about transaction isolation, but doesn't go as far
as mentioning anomalies and the individual isolation levels.

Perhaps you feel inspired to write a patch for that page that demonstrates
the lost update and shows how to avoid it using the REPEATABLE READ
isolation level?

Yours,
Laurenz Albe