Re: Is a pg_stat_force_next_flush() call sufficient for regression tests?

At Mon, 3 Jul 2023 15:45:52 +0200, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> wrote in
> So I'm wondering if pg_stat_force_next_flush() is enough - AFAICS this
> only sets some flag for the *next* pgstat_report_stat() call, but how do
> we know that happens before the query execution?
>
> Shouldn't there be something like pg_stat_flush() that actually does the
> flushing, instead of just setting the flag?

The reason for the function is that pg_stat_flush() is supposed not to
be called within a transaction. AFAICS pg_stat_force_next_flush()
takes effect after a successfull transaction end and before the next
command execution.

To verify this, I put in an assertion to check that the flag gets
consumed before reading of pg_stat_io (a.diff), then ran pgbench with
the attached custom script. As expected, it didn't fire at all during
several trials. When I wrapped all lines in t.sql within a
begin-commit block, the assertion fired off immediately as a matter of
course.

Is there any chance concurrent backends or some other things can
actually hinder the backend from reusing buffers?

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center