Thread

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

   PG Bug reporting form <noreply@postgresql.org> — 2025-10-02T13:11:37Z

   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.
   
   

2. Re: BUG #19069: pg_advisory_xact_lock() in a WITH query doesn't work

   Tom Lane <tgl@sss.pgh.pa.us> — 2025-10-02T20:13:55Z

   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
   
   
   
   

3. RE: BUG #19069: pg_advisory_xact_lock() in a WITH query doesn't work

   Maciej Walczak <m.walczak@megavision.pl> — 2025-10-03T09:34:39Z

   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