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

On Mon, Aug 22, 2022 at 4:42 AM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
>
> On Fri, Aug 19, 2022 at 7:55 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Fri, Aug 19, 2022 at 3:05 PM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
> > >
> > > On Fri, Aug 19, 2022 at 7:10 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > >
> > > > On Fri, Aug 19, 2022 at 2:36 PM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
> > > > >
> > > > > Here are my review comments for the v23-0005 patch:
> > > > >
> > > > > ======
> > > > >
> > > > > Commit Message says:
> > > > > main_worker_pid is Process ID of the main apply worker, if this process is a
> > > > > apply background worker. NULL if this process is a main apply worker or a
> > > > > synchronization worker.
> > > > > The new column can make it easier to distinguish main apply worker and apply
> > > > > background worker.
> > > > >
> > > > > --
> > > > >
> > > > > Having a column called ‘main_worker_pid’ which is defined to be NULL
> > > > > if the process *is* the main apply worker does not make any sense to
> > > > > me.
> > > > >
> > > >
> > > > I haven't read this part of a patch but it seems to me we have
> > > > something similar for parallel query workers. Refer 'leader_pid'
> > > > column in pg_stat_activity.
> > > >
> > >
> > > IIUC (from the patch 0005 commit message) the intention is to be able
> > > to easily distinguish the worker types.
> > >
> >
> > I think it is only to distinguish between leader apply worker and
> > background apply workers. The tablesync worker can be distinguished
> > based on relid field.
> >
>
> Right. But that's the reason for my question in the first place - why
> implement the patch so that the user still has to jump through hoops
> just to know the worker type information?
>

I think it is not only to judge worker type but also to know the pid
of each of the workers during parallel apply. Isn't it better to have
both main apply worker pid and parallel apply worker pid as we have
for the parallel query system?

--
With Regards,
Amit Kapila.