Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)

On Tue, Apr 4, 2023 at 4:35 PM Pavel Luzanov <p(dot)luzanov(at)postgrespro(dot)ru> wrote:
>
> On 03.04.2023 23:50, Melanie Plageman wrote:
> > Attached is a tiny patch to add standalone backend type to
> > pg_stat_activity documentation (referenced by pg_stat_io).
> >
> > I mentioned both the bootstrap process and single user mode process in
> > the docs, though I can't imagine that the bootstrap process is relevant
> > for pg_stat_activity.
>
> After a little thought... I'm not sure about the term 'bootstrap
> process'. I can't find this term in the documentation.

There are various mentions of "bootstrap" peppered throughout the docs
but no concise summary of what it is. For example, initdb docs mention
the "bootstrap backend" [1].

Interestingly, 910cab820d0 added "Bootstrap superuser" in November. This
doesn't really cover what bootstrapping is itself, but I wonder if that
is useful? If so, you could propose a glossary entry for it?
(preferably in a new thread)

> Do I understand correctly that this is a postmaster? If so, then the
> postmaster process is not shown in pg_stat_activity.

No, bootstrap process is for initializing the template database. You
will not be able to see pg_stat_activity when it is running.

> Perhaps it may be worth adding a description of the standalone backend
> to pg_stat_io, not to pg_stat_activity.
> Something like: backend_type is all types from pg_stat_activity plus
> 'standalone backend',
> which is used for the postmaster process and in a single user mode.

You can query pg_stat_activity from single user mode, so it is relevant
to pg_stat_activity also. I take your point that bootstrap mode isn't
relevant for pg_stat_activity, but I am hesitant to add that distinction
to the pg_stat_io docs since the reason you won't see it in
pg_stat_activity is because it is ephemeral and before a user can access
the database and not because stats are not tracked for it.

Can you think of a way to convey this?

> > I also noticed that the pg_stat_activity docs call background workers
> > "parallel workers" (though it also mentions that extensions could have
> > other background workers registered), but this seems a bit weird because
> > pg_stat_activity uses GetBackendTypeDesc() and this prints "background
> > worker" for type B_BG_WORKER. Background workers doing parallelism tasks
> > is what users will most often see in pg_stat_activity, but I feel like
> > it is confusing to have it documented as something different than what
> > would appear in the view. Unless I am misunderstanding something...
>
> 'parallel worker' appears in the pg_stat_activity for parallel queries.
> I think it's right here.

Ah, I didn't read the code closely enough in pg_stat_get_activity()
Even though there is no BackendType which GetBackendTypeDesc() returns
called "parallel worker", we to out of our way to be specific using
GetBackgroundWorkerTypeByPid()

/* Add backend type */
if (beentry->st_backendType == B_BG_WORKER)
{
const char *bgw_type;

bgw_type = GetBackgroundWorkerTypeByPid(beentry->st_procpid);
if (bgw_type)
values[17] = CStringGetTextDatum(bgw_type);
else
nulls[17] = true;
}
else
values[17] =
CStringGetTextDatum(GetBackendTypeDesc(beentry->st_backendType));

- Melanie

[1] https://www.postgresql.org/docs/current/app-initdb.html