Re: Replication slot stats misgivings

On Mon, Apr 19, 2021 at 2:14 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Mon, Apr 19, 2021 at 9:00 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Fri, Apr 16, 2021 at 2:58 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > >
> > > 4.
> > > +CREATE VIEW pg_stat_replication_slots AS
> > > + SELECT
> > > + s.slot_name,
> > > + s.spill_txns,
> > > + s.spill_count,
> > > + s.spill_bytes,
> > > + s.stream_txns,
> > > + s.stream_count,
> > > + s.stream_bytes,
> > > + s.total_txns,
> > > + s.total_bytes,
> > > + s.stats_reset
> > > + FROM pg_replication_slots as r,
> > > + LATERAL pg_stat_get_replication_slot(slot_name) as s
> > > + WHERE r.datoid IS NOT NULL; -- excluding physical slots
> > > ..
> > > ..
> > >
> > > -/* Get the statistics for the replication slots */
> > > +/* Get the statistics for the replication slot */
> > > Datum
> > > -pg_stat_get_replication_slots(PG_FUNCTION_ARGS)
> > > +pg_stat_get_replication_slot(PG_FUNCTION_ARGS)
> > > {
> > > #define PG_STAT_GET_REPLICATION_SLOT_COLS 10
> > > - ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
> > > + text *slotname_text = PG_GETARG_TEXT_P(0);
> > > + NameData slotname;
> > >
> > > I think with the above changes getting all the slot stats has become
> > > much costlier. Is there any reason why can't we get all the stats from
> > > the new hash_table in one shot and return them to the user?
> >
> > I think the advantage of this approach would be that it can avoid
> > showing the stats for already-dropped slots. Like other statistics
> > views such as pg_stat_all_tables and pg_stat_all_functions, searching
> > the stats by the name got from pg_replication_slots can show only
> > available slot stats even if the hash table has garbage slot stats.
> >
>
> Sounds reasonable. However, if the create_slot message is missed, it
> will show an empty row for it. See below:
>
> postgres=# select slot_name, total_txns from pg_stat_replication_slots;
> slot_name | total_txns
> -----------+------------
> s1 | 0
> s2 | 0
> |
> (3 rows)
>
> Here, I have manually via debugger skipped sending the create_slot
> message for the third slot and we are showing an empty for it. This
> won't happen for pg_stat_all_tables, as it will set 0 or other initial
> values in such a case. I think we need to address this case.

Good catch. I think it's better to set 0 to all counters and NULL to
reset_stats.

>
> > Given that pg_stat_replication_slots doesn’t show garbage slot stats
> > even if it has, I thought we can avoid checking those garbage stats
> > frequently. It should not essentially be a problem for the hash table
> > to have entries up to max_replication_slots regardless of live or
> > already-dropped.
> >
>
> Yeah, but I guess we still might not save much by not doing it,
> especially because for the other cases like tables/functions, we are
> doing it without any threshold limit.

Agreed.

I've attached the updated patch, please review it.

Regards,

--
Masahiko Sawada
EDB: https://www.enterprisedb.com/