From: | "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andres Freund <andres(at)anarazel(dot)de>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: parallel.c is not marked as test covered |
Date: | 2016-05-11 17:48:41 |
Message-ID: | CAKFQuwZAjVxzhMow_oE5jaEs_zrGnEAqr7euJWan9JJcP3H8Qw@mail.gmail.com |
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On Wed, May 11, 2016 at 10:38 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Wed, May 11, 2016 at 12:34 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> Hmm, that is strange. I would have expected that to stuff a Gather on
> >> top of the Aggregate. I wonder why it's not doing that.
> >
> > The reason is that create_plain_partial_paths() contains a hard-wired
> > decision not to generate any partial paths for relations smaller than
> > 1000 blocks, which means that no partial path will ever be generated
> > for *any* relation in the standard regression tests, force_parallel_mode
> > or no.
>
> Well that's an interesting theory, except that you've completely
> missed the point of force_parallel_mode. force_parallel_mode pushes a
> special Gather node on top of any plan that is not already parallel in
> some way but which is parallel-safe. That special Gather node runs
> only in the worker, not the leader, and always uses just one worker.
>
What happens when there are no workers available due to
max_worker_processes already being assigned?
Related question, if max_parallel_degree is >1 and "the requested number of
workers may not actually be available at runtime" is true, does the degree
of parallelism minimize at 1 worker + leader or will the leader simply run
the query by itself?
David J.
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