Horton

Re: per backend WAL statistics

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: Andres Freund <andres@anarazel.de>, pgsql-hackers@lists.postgresql.org, Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: 2025-02-28T09:26:08Z
Lists: pgsql-hackers

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Hi,

On Fri, Feb 28, 2025 at 11:34:14AM +0900, Michael Paquier wrote:
> On Thu, Feb 27, 2025 at 07:47:09AM +0000, Bertrand Drouvot wrote:
> > That's how I did it initially but decided to move it to pgstat_backend.c. The
> > reason was that it's fully linked to "per backend" stats and that there is
> > no SQL api on top of it (while I think that's the case for almost all the ones
> > in pgstatfuncs.c). Thoughts?
> 
> Okay by me with pgstat_fetch_stat_backend in parallel, why not
> exposing this part as well..  Perhaps that could be useful for some
> extension?  I'd rather have out-of-core code do these lookups with the
> same sanity checks in place for the procnumber and slot lookups.

Yeah that's also a pros for it.

> The name was inconsistent with the rest of the file, so I have settled
> to a pgstat_fetch_stat_backend_by_pid() to be more consistent.

Sounds good, thanks!

> A
> second thing is to properly initialize bktype if defined by the
> caller.

Saw that in c2a50ac678e, makes sense.

> Attached is a rebased version of the rest.

The rebased version looks ok.

Also attaching the patch I mentioned up-thread to address some of Rahila's
comments ([1]): It adds a AuxiliaryPidGetProc() call in pgstat_fetch_stat_backend_by_pid()
and pg_stat_reset_backend_stats(). I think that fully makes sense since a051e71e28a
modified pgstat_tracks_backend_bktype() for B_WAL_RECEIVER, B_WAL_SUMMARIZER
and B_WAL_WRITER.

It looks like it does not need doc updates. Attached as 0002 as it's somehow
un-related to this thread (but not sure it deserves it's dedicated thread though).

[1]: https://www.postgresql.org/message-id/CAH2L28v9BwN8_y0k6FQ591=0g2Hj_esHLGj3bP38c9nmVykoiA@mail.gmail.com
[2]: https://www.postgresql.org/message-id/flat/Z8FMjlyNpNicucGa%40paquier.xyz#92d3e2b9ad860708f76b8f9c6f49f32d

Regards,

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

Commits

  1. Fix use-after-free in pgstat_fetch_stat_backend_by_pid()

  2. Remove initialization from PendingBackendStats

  3. Add WAL data to backend statistics

  4. Improve check for detection of pending data in backend statistics

  5. Fix some gaps in pg_stat_io with WAL receiver and WAL summarizer

  6. Handle auxiliary processes in SQL functions of backend statistics

  7. Invent pgstat_fetch_stat_backend_by_pid()

  8. Refactor code of pg_stat_get_wal() building result tuple

  9. Adding new PgStat_WalCounters structure in pgstat.h

  10. Remove pgstat_flush_wal()

  11. Refactor some code related to backend statistics

  12. Add backend-level statistics to pgstats