Re: Resetting spilled txn statistics in pg_stat_replication

On Tue, Oct 13, 2020 at 9:02 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Oct 13, 2020 at 4:54 AM Masahiko Sawada
> <masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
> >
> > Attached the updated version patch. Please review it.
> >
>
> I have pushed this but it failed in one of the BF. See
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=florican&dt=2020-10-13%2003%3A07%3A25
>
> The failure is shown below and I am analyzing it. See, if you can
> provide any insights.
>
> @@ -58,7 +58,7 @@
> SELECT name, spill_txns, spill_count FROM pg_stat_replication_slots;
> name | spill_txns | spill_count
> -----------------+------------+-------------
> - regression_slot | 1 | 12
> + regression_slot | 1 | 10
> (1 row)
>
> -- reset the slot stats, and wait for stats collector to reset
> @@ -96,7 +96,7 @@
> SELECT name, spill_txns, spill_count FROM pg_stat_replication_slots;
> name | spill_txns | spill_count
> -----------------+------------+-------------
> - regression_slot | 1 | 12
> + regression_slot | 1 | 10
> (1 row)
>

The reason for this problem could be that there is some transaction
(say by autovacuum) which happened interleaved with this transaction
and committed before this one. Now during DecodeCommit of this
background transaction, we will send the stats accumulated by that
time which could lead to such a problem.

--
With Regards,
Amit Kapila.