Re: per backend WAL statistics

Hi,

On Wed, Feb 26, 2025 at 04:52:13PM +0900, Michael Paquier wrote:
> On Tue, Feb 25, 2025 at 03:00:35PM +0000, Bertrand Drouvot wrote:
> > That makes fully sense. Done in 0004 attached. Somehow related to that, I've
> > a patch in progress to address some of Rahila's comments ([1]) (the one related
> > to the AuxiliaryPidGetProc() call is relevant specially since a051e71e28a where
> > pgstat_tracks_backend_bktype() has been modified for B_WAL_RECEIVER, B_WAL_SUMMARIZER
> > and B_WAL_WRITER). I'll wait for 0004 to go in before sharing the patch.
>
> Applied v9-0001

I see that you removed pgstat_flush_wal() in d7cbeaf261d (instead of what 0001
was doing i.e making it static). Makes sense to me.a

> and v9-0003 as these were fine,

Thanks.

> with more
> documentation added in pgstat.h for the new WAL structure, and the
> reason why it exists.

Saw that, looks good.

> I've noticed the difference with bktype in
> v9-0004 as the WAL part does not need this information when generating
> its tuple, OK here.

Thx.

> Doing v9-0003 after v9-0002 felt a bit odd, changing twice the
> signature of pg_stat_wal_build_tuple() to adapt with the split for the
> reset timestamp.

PFA a rebase.

> - values[4] = TimestampTzGetDatum(wal_stats->stat_reset_timestamp);
> + if (wal_stats.stat_reset_timestamp != 0)
> + values[4] = TimestampTzGetDatum(wal_stats.stat_reset_timestamp);
> + else
> + nulls[4] = true;
>
> In patch v9-0002, is this nulls[4] required for the backend part?

Yup. That's what we've done in pg_stat_io_build_tuples() too (ff7c40d7fd6).
Without this we'd get "2000-01-01 00:00:00+00" in the stats_reset field of
pg_stat_get_backend_wal() and pg_stat_get_backend_io().

That was not needed for pg_stat_io and pg_stat_wal because the stats_reset field
was already non null after initdb.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com