Re: "actual time" in QUERY PLAN for parallel operation when loops is bigger than 1

Hi,

Le mer. 17 nov. 2021 à 06:32, tanghy(dot)fnst(at)fujitsu(dot)com <
tanghy(dot)fnst(at)fujitsu(dot)com> a écrit :

> Wednesday, November 17, 2021 6:29 AM, nikolai.berkoff <
> nikolai(dot)berkoff(at)pm(dot)me> wrote:
> >
> > Parallel query is explained in
> > https://www.postgresql.org/docs/14/how-parallel-query-works.html and
> > https://www.postgresql.org/docs/14/parallel-plans.html
> >
> > The docs seem clear to me that as the nodes are executed in parallel
> then the
> > time execution time is not 140.036*3. The 140.036 value is actual time
> the Parallel
> > Seq Scan nodes ran for but there were up to 2 running in parallel.
>
> Thanks for your reply.
> I read your references but still confused about the 'loops' in parallel
> query result.
>
> > -> Parallel Seq Scan on c (cost=0.00..8591.67 rows=416667 width=0)
> (actual time=0.030..140.036 rows=333333 loops=3)
> In my previous example, actual row number is 333333*3=1e6(which is
> correct), so I think the actual time is 140.036*3ms.
> Do your think the loops(3) has no meaning for parallel scan node when
> calculate actual time?
>
>
As far as I understand it, you have to multiply the number of rows by the
number of loops, but this doesn't apply to duration at least for parallel
queries.

--
Guillaume.