Обсуждение: BUG #19069: pg_advisory_xact_lock() in a WITH query doesn't work

The following bug has been logged on the website:

Bug reference:      19069
Logged by:          Maciej Walczak
Email address:      m.walczak@megavision.pl
PostgreSQL version: 18.0
Operating system:   Windows 11 Pro 24H2
Description:

This doesn't actually do any locking:

BEGIN;
WITH my_lock AS (
        SELECT pg_advisory_xact_lock(1)
)
INSERT INTO my_table (...) VALUES (...);
COMMIT;

To reproduce just run this code in two sessions in parallel without
commiting.
Both inserts will immediately complete without blocking.

I'm not sure if this is a bug. If it's not, perhaps the documentation should
be clarified.
Side-effects of WITH queries are mentioned here, so I would expect my code
to work:
https://www.postgresql.org/docs/16/queries-with.html#QUERIES-WITH-CTE-MATERIALIZATION

Tested on Postgres 18.0 and 16.10.

PG Bug reporting form <noreply@postgresql.org> writes:
> This doesn't actually do any locking:

> BEGIN;
> WITH my_lock AS (
>         SELECT pg_advisory_xact_lock(1)
> )
> INSERT INTO my_table (...) VALUES (...);
> COMMIT;

That looks as-expected to me.  The docs say [1]

    [ This works because ] PostgreSQL's implementation evaluates only
    as many rows of a WITH query as are actually fetched by the parent
    query.

which is to say, none at all in this case.  There's also this in [2]:

    Data-modifying statements in WITH are executed exactly once, and
    always to completion, independently of whether the primary query
    reads all (or indeed any) of their output. Notice that this is
    different from the rule for SELECT in WITH: as stated in the
    previous section, execution of a SELECT is carried only as far as
    the primary query demands its output.

You're apparently expecting the presence of a volatile function
to cause the SELECT to be reclassified as data-modifying, but
we don't do it that way.

I'd recommend being less cute and just writing

BEGIN;
SELECT pg_advisory_xact_lock(1);
INSERT INTO my_table (...) VALUES (...);
COMMIT;

            regards, tom lane

[1] https://www.postgresql.org/docs/current/queries-with.html#QUERIES-WITH-CYCLE

[2] https://www.postgresql.org/docs/current/queries-with.html#QUERIES-WITH-MODIFYING

Thank you.
I missed those parts of the documentation. I should have studied it more carefully.

The reason I tried to use WITH is that a separate

SELECT pg_advisory_xact_lock(1);

statement confuses my object mapper (Dapper) when I want to use a RETURNING clause in the INSERT.
I found a workaround:

DO $$ BEGIN
    PERFORM pg_advisory_xact_lock(1);
END $$;

Regards
Maciej Walczak

-----Original Message-----
From: Tom Lane <tgl@sss.pgh.pa.us>
Sent: 02 October 2025 22:14
To: Maciej Walczak <m.walczak@megavision.pl>
Cc: pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #19069: pg_advisory_xact_lock() in a WITH query doesn't work

PG Bug reporting form <noreply@postgresql.org> writes:
> This doesn't actually do any locking:

> BEGIN;
> WITH my_lock AS (
>         SELECT pg_advisory_xact_lock(1)
> )
> INSERT INTO my_table (...) VALUES (...); COMMIT;

That looks as-expected to me.  The docs say [1]

    [ This works because ] PostgreSQL's implementation evaluates only
    as many rows of a WITH query as are actually fetched by the parent
    query.

which is to say, none at all in this case.  There's also this in [2]:

    Data-modifying statements in WITH are executed exactly once, and
    always to completion, independently of whether the primary query
    reads all (or indeed any) of their output. Notice that this is
    different from the rule for SELECT in WITH: as stated in the
    previous section, execution of a SELECT is carried only as far as
    the primary query demands its output.

You're apparently expecting the presence of a volatile function to cause the SELECT to be reclassified as
data-modifying,but we don't do it that way. 

I'd recommend being less cute and just writing

BEGIN;
SELECT pg_advisory_xact_lock(1);
INSERT INTO my_table (...) VALUES (...); COMMIT;

            regards, tom lane

[1] https://www.postgresql.org/docs/current/queries-with.html#QUERIES-WITH-CYCLE

[2] https://www.postgresql.org/docs/current/queries-with.html#QUERIES-WITH-MODIFYING