Re: Resetting spilled txn statistics in pg_stat_replication

On Tue, Oct 13, 2020 at 9:11 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> 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.
>

If this theory is correct then I think we can't rely on the
'spill_count' value, what do you think?

--
With Regards,
Amit Kapila.