Re: Expose lock group leader pid in pg_stat_activity

Le jeu. 26 déc. 2019 à 09:49, Julien Rouhaud <rjuju123(at)gmail(dot)com> a écrit :

> On Thu, Dec 26, 2019 at 9:08 AM Guillaume Lelarge
> <guillaume(at)lelarge(dot)info> wrote:
> >
> > Le mer. 25 déc. 2019 à 19:30, Julien Rouhaud <rjuju123(at)gmail(dot)com> a
> écrit :
> >>
> >> On Wed, Dec 25, 2019 at 7:03 PM Julien Rouhaud <rjuju123(at)gmail(dot)com>
> wrote:
> >> >
> >> > Guillaume (in Cc) recently pointed out [1] that it's currently not
> >> > possible to retrieve the list of parallel workers for a given backend
> >> > at the SQL level. His use case was to develop a function in plpgsql
> >> > to sample a given query wait event, but it's not hard to imagine other
> >> > useful use cases for this information, for instance doing some
> >> > analysis on the average number of workers per parallel query, or ratio
> >> > of parallel queries. IIUC parallel queries is for now the only user
> >> > of lock group, so this should work just fine.
> >> >
> >> > I'm attaching a trivial patch to expose the group leader pid if any
> >> > in pg_stat_activity. Quick example of usage:
> >> >
> >> > =# SELECT query, leader_pid,
> >> > array_agg(pid) filter(WHERE leader_pid != pid) AS members
> >> > FROM pg_stat_activity
> >> > WHERE leader_pid IS NOT NULL
> >> > GROUP BY query, leader_pid;
> >> > query | leader_pid | members
> >> > -------------------+------------+---------------
> >> > select * from t1; | 28701 | {28728,28732}
> >> > (1 row)
> >> >
> >> >
> >> > [1] https://twitter.com/g_lelarge/status/1209486212190343168
> >>
> >> And I just realized that I forgot to update rule.out, sorry about
> >> that. v2 attached.
> >
> >
> > So I tried your patch this morning, and it works really well.
> >
> > On a SELECT count(*), I got this:
> >
> > SELECT leader_pid, pid, wait_event_type, wait_event, state, backend_type
> FROM pg_stat_activity WHERE pid=111439 or leader_pid=111439;
> >
> >
> ┌────────────┬────────┬─────────────────┬──────────────┬────────┬─────────────────┐
> > │ leader_pid │ pid │ wait_event_type │ wait_event │ state │
> backend_type │
> >
> ├────────────┼────────┼─────────────────┼──────────────┼────────┼─────────────────┤
> > │ 111439 │ 111439 │ LWLock │ WALWriteLock │ active │ client
> backend │
> > │ 111439 │ 116887 │ LWLock │ WALWriteLock │ active │
> parallel worker │
> > │ 111439 │ 116888 │ IO │ WALSync │ active │
> parallel worker │
> >
> └────────────┴────────┴─────────────────┴──────────────┴────────┴─────────────────┘
> > (3 rows)
> >
> > and this from a CREATE INDEX:
> >
> >
> ┌────────────┬────────┬─────────────────┬────────────┬────────┬─────────────────┐
> > │ leader_pid │ pid │ wait_event_type │ wait_event │ state │
> backend_type │
> >
> ├────────────┼────────┼─────────────────┼────────────┼────────┼─────────────────┤
> > │ 111439 │ 111439 │ │ │ active │ client
> backend │
> > │ 111439 │ 118775 │ │ │ active │ parallel
> worker │
> >
> └────────────┴────────┴─────────────────┴────────────┴────────┴─────────────────┘
> > (2 rows)
> >
> > Anyway, it applies cleanly, it compiles, and it works. Documentation is
> available. So it looks to me it's good to go :)
>
> Thanks for the review Guillaume. Double checking the doc, I see that
> I made a copy/pasto mistake in the new field name. Attached v3 should
> be all good.
>

Feeling bad I missed this. But, yeah, it's much better with the right
column's name.

For me, it's looking good to be ready for commiter. Should I set it this
way in the Commit Fest app?

--
Guillaume.