Re: Warn against transaction level advisory locks for emulating "flat file" DBMS locking strategies

On Wed, 2021-09-08 at 08:23 +0000, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/13/explicit-locking.html
> Description:
>
> The docs mention "For example, a common use of advisory locks is to emulate
> pessimistic locking strategies typical of so-called “flat file” data
> management systems" which is exactly what I wanted to use to port some code
> from using SQLite to using PostgreSQL. (The code in question requires
> serializable transactions and cannot not handle retries.)
>
> The next paragraph explains session and transaction level advisory locks and
> mentions that transaction level locks are "often more convenient than the
> session-level behavior". This seemed to be true for this use case, so I
> chose to use them.
>
> Later I discovered that obtaining a transaction level lock as first
> statement _within_ a transaction is not sufficient to emulate global
> pessimistic locking and can occasionally still result in serialization
> failures.

I don't see how that is related to session-level vs. transaction level locks.

erhaps you can explain your case in some more detail.

Yours,
Laurenz Albe
--
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