Re: RFC: Logging plan of the running query

On Mon, Oct 20, 2025 at 8:15 AM torikoshia <torikoshia@oss.nttdata.com> wrote:
> > S1: Set an injection point to wait in HandleLogQueryPlanInterrupt
> > [runs to completion].
> > S2: Take an advisory lock [runs to completion].
> > S1: Start a query that attempts to acquire the same advisory lock.
> > Use $node->wait_for_event() to be sure that S1 is now waiting on the
> > lock.
> > S2: Commit, thus releasing the advisory lock [runs to completion].
> > Use $node->wait_for_event() to be sure that S1 is now waiting inside
> > HandleLogQueryPlanInterrupt.
> > Remember the log offset, as in
> > src/test/modules/test_misc/t/005_timeouts.pl
> > Detach the injection point.
> > Use $node->wait_for_log() to wait for the expected log message to
> > appear.
> >
> > It's really hard to make these kinds of sequences race-free, so there
> > could well be bugs in the above outline, but I hope it is close to the
> > right thing.
>
> Thanks for the detailed outline!
> Attached patch adds a test following the suggestion.

Thanks. I'm not sure about this part:

+# Run pg_log_query_plan().
+# Then commit the session 2 to release the advisory lock.
+$psql_session2->query_safe(
+ qq[
+ SELECT pg_log_query_plan($session1_pid);
+ COMMIT;
+]);

This does two relevant things: one is to send a signal (the SELECT)
and the other is to release a lock (the COMMIT). Both of these events
will soon be perceived by the other session, but I am not sure whether
we have a guarantee about the order. If it's possible for the lock
release to be observed by the target session before the log-query-plan
interrupt arrives, the test will fail. If that is possible, I think we
should try to find a way to plug the gap. If it's not possible, I
think we should add a comment explaining why it can't happen.

Also, my apologies for taking some time to return to this thread.

Thanks,

--
Robert Haas
EDB: http://www.enterprisedb.com