Re: Resetting spilled txn statistics in pg_stat_replication

On Tue, Jun 23, 2020 at 3:48 PM Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
>
> On Tue, Jun 23, 2020 at 10:58:18AM +0530, Amit Kapila wrote:
> >On Tue, Jun 23, 2020 at 9:32 AM Masahiko Sawada
> ><masahiko.sawada@2ndquadrant.com> wrote:
> >>
> >> On Sun, 21 Jun 2020 at 06:57, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
> >> >
> >> > >
> >> > >What if the decoding has been performed by multiple backends using the
> >> > >same slot?  In that case, it will be difficult to make the judgment
> >> > >for the value of logical_decoding_work_mem based on stats.  It would
> >> > >make sense if we provide a way to set logical_decoding_work_mem for a
> >> > >slot but not sure if that is better than what we have now.
> >> > >
> >>
> >> I thought that the stats are relevant to what
> >> logical_decoding_work_mem value was but not with who performed logical
> >> decoding. So even if multiple backends perform logical decoding using
> >> the same slot, the user can directly use stats as long as
> >> logical_decoding_work_mem value doesn’t change.
> >>
> >
> >I think if you maintain these stats at the slot level, you probably
> >need to use spinlock or atomic ops in order to update those as slots
> >can be used from multiple backends whereas currently, we don't need
> >that.
>
> IMHO storing the stats in the slot itself is a bad idea. We have the
> statistics collector for exactly this purpose, and it's receiving data
> over UDP without any extra locking etc.
> >
> >> > >What problems do we see in displaying these for each process?  I think
> >> > >users might want to see the stats for the exited processes or after
> >> > >server restart but I think both of those are not even possible today.
> >> > >I think the stats are available till the corresponding WALSender
> >> > >process is active.
> >>
> >> I might want to see the stats for the exited processes or after server
> >> restart. But I'm inclined to agree with displaying the stats per
> >> process if the stats are displayed on a separate view (e.g.
> >> pg_stat_replication_slots).
> >>
> >
> >Yeah, as told previously, this makes more sense to me.
> >
> >Do you think we should try to write a POC patch using a per-process
> >entry approach and see what difficulties we are facing and does it
> >give the stats in a way we are imagining but OTOH, we can wait for
> >some more to see if there is clear winner approach here?
> >
>
> I may be missing something obvious, but I still see no point in tracking
> per-process stats. We don't have that for other stats,
>

Won't we display per-process information in pg_stat_replication?
These stats are currently displayed in that view and one of the
shortcomings was that it won't display these stats when we decode via
backend.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com