Re: per backend WAL statistics

On Tue, Feb 04, 2025 at 08:49:41AM +0000, Bertrand Drouvot wrote:
> I think that:
>
> wal_write (and wal_write_time)
> wal_sync (and wal_sync_time)

Right. We are not able to get this data from XLogWrite() and
issue_xlog_fsync(), so there is no need to duplicate that anymore in
your patch.

> can be extracted from pg_stat_get_backend_io(), so there is no need to duplicate
> this information. The same comment could be done for pg_stat_wal and pg_stat_io
> though, but pg_stat_wal already exists so removing fields has not the same
> effect.
>
> What are you thoughts about keeping in pg_stat_get_backend_wal() only the
> 4 stats mentioned above?

wal_buffers_full is incremented in AdvanceXLInsertBuffer(), part of
PendingWalStats. wal_records, wal_fpi and wal_bytes are part of the
instrumentation field. It looks to me that if you discard the
wal_buffers_full part, the implementation of the data in the backend
could just be tied to the fields coming from WalUsage.

Actually, could it actually be useful to have wal_buffers_full be
available in WalUsage, so as it would show up in EXPLAIN in a
per-query basis with show_wal_usage()? Consolidating that would make
what you are trying it a bit easier, because we would have the
WalUsage and the pg_stat_io parts without any of the PendingWalStats
part. And it is just a counter not that expensive to handle, like the
data for records, fpis and bytes. This extra information could be
useful to have in the context of an EXPLAIN.
--
Michael