Re: Fix cumulative WAL statistics double-counting from parallel workers

Hi Shihao,

On Mon, Nov 09, 2026 at 09:51:13PM -0500, shihao zhong wrote:
> Hi Lukas,
>
> Reading v1, I think the pgstat_backend.c part makes the leader drop
> the WAL of its workers from pg_stat_get_backend_wal(). A worker drops
> its own backend stats entry when it exits. So on master the leader
> entry is the only place that counts that WAL, and it counts it once.
>
> I checked it with your parallel VACUUM case. On master I get
>
> actual_wal_bytes | pg_stat_wal_bytes | leader_backend_wal_bytes
> 95926320 | 128524537 | 97581712
>
> With v1 I get
>
> actual_wal_bytes | pg_stat_wal_bytes | leader_backend_wal_bytes
> 97900592 | 97581712 | 66638887
>
> So pg_stat_wal is right with v1, but the leader entry lost about
> 31MB. Is leader only intended here, to match pg_stat_get_backend_io()?

Yeah, I think so. The documentation says that pg_stat_get_backend_wal() reports
statistics about the backend with the specified process ID, so worker's WAL should
not be attributed to the leader.

So I think the pgstat_backend.c changes should stay, but the commit message
should probably clarify that this part fixes attribution rather than double
counting.

Regards,

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