RE: Perform streaming logical transactions by background workers and parallel apply

On Thursday, January 12, 2023 7:08 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Thu, Jan 12, 2023 at 4:21 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > On Thu, Jan 12, 2023 at 10:34 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> wrote:
> > >
> > > On Thu, Jan 12, 2023 at 9:54 AM Peter Smith <smithpb2250(at)gmail(dot)com>
> wrote:
> > > >
> > > >
> > > > doc/src/sgml/monitoring.sgml
> > > >
> > > > 5. pg_stat_subscription
> > > >
> > > > @@ -3198,11 +3198,22 @@ SELECT pid, wait_event_type, wait_event
> > > > FROM pg_stat_activity WHERE wait_event i
> > > >
> > > > <row>
> > > > <entry role="catalog_table_entry"><para
> > > > role="column_definition">
> > > > + <structfield>apply_leader_pid</structfield>
> <type>integer</type>
> > > > + </para>
> > > > + <para>
> > > > + Process ID of the leader apply worker, if this process is a apply
> > > > + parallel worker. NULL if this process is a leader apply worker or a
> > > > + synchronization worker.
> > > > + </para></entry>
> > > > + </row>
> > > > +
> > > > + <row>
> > > > + <entry role="catalog_table_entry"><para
> > > > + role="column_definition">
> > > > <structfield>relid</structfield> <type>oid</type>
> > > > </para>
> > > > <para>
> > > > OID of the relation that the worker is synchronizing; null for the
> > > > - main apply worker
> > > > + main apply worker and the parallel apply worker
> > > > </para></entry>
> > > > </row>
> > > >
> > > > 5a.
> > > >
> > > > (Same as general comment #1 about terminology)
> > > >
> > > > "apply_leader_pid" --> "leader_apply_pid"
> > > >
> > >
> > > How about naming this as just leader_pid? I think it could be
> > > helpful in the future if we decide to parallelize initial sync (aka
> > > parallel
> > > copy) because then we could use this for the leader PID of parallel
> > > sync workers as well.
> > >
> > > --
> >
> > I still prefer leader_apply_pid.
> > leader_pid does not tell which 'operation' it belongs to. 'apply'
> > gives the clarity that it is apply related process.
> >
>
> But then do you suggest that tomorrow if we allow parallel sync workers then
> we have a separate column leader_sync_pid? I think that doesn't sound like a
> good idea and moreover one can refer to docs for clarification.

I agree that leader_pid would be better not only for future parallel copy sync feature,
but also it's more consistent with the leader_pid column in pg_stat_activity.

And here is the version patch which addressed Peter's comments and renamed all
the related stuff to leader_pid.

Best Regards,
Hou zj