Horton

Re: per backend WAL statistics

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Alexander Lakhin <exclusion@gmail.com>, Andres Freund <andres@anarazel.de>, Xuneng Zhou <xunengzhou@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2025-04-07T00:54:46Z
Lists: pgsql-hackers 
On Mon, Mar 31, 2025 at 07:42:19AM +0000, Bertrand Drouvot wrote:
> I think we can simply move the pgstat_fetch_stat_backend() call at the end
> of pgstat_fetch_stat_backend_by_pid(), like in the attached. With this in place
> the issue is fixed on my side.
> 
> Thoughts?

Confirmed.  I agree that it is simpler to move all the accesses to
beentry before attempting to retrieve the pgstats entry.

One thing that itched me a bit in the patch is that we would set
bktype even if we don't have a pgstats entry.  The two callers of
pgstat_fetch_stat_backend_by_pid() return tuples full of NULLs and
zeros in this case, discarding the backend type automatically, but
let's keep the API consistent and set the value to B_INVALID if
pgstat_fetch_stat_backend() returns NULL.

I have added a comment warning about not accessing beentry when
fetching the backend pgstats entry, and applied the result.  Thanks
for the report, Alexander, and for the patch, Bertrand.
--
Michael

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