Re: Expose lock group leader pid in pg_stat_activity

On Thu, Dec 26, 2019 at 10:20 AM Guillaume Lelarge
<guillaume(at)lelarge(dot)info> wrote:
>
> 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?

If you don't see any other issue with the patch, I'd say yes. A
committer can still put it back to waiting on author if needed.