From: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: print_path is missing GatherMerge and CustomScan support
Date: Fri, 27 Jul 2018 15:40:48 +0900
Message-ID: <5B5ABE70.40606@lab.ntt.co.jp> (raw)
In-Reply-To: <CA+TgmoaDYiXj6mLZ0Cg3nN3yPgpjxQ+8sjanxRdKo1nTan9dXA@mail.gmail.com>
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(2018/07/27 4:50), Robert Haas wrote:
> On Thu, Jul 26, 2018 at 1:14 AM, Etsuro Fujita
> <fujita.etsuro@lab.ntt.co.jp> wrote:
>> because we currently don't consider gathering partial child-scan or
>> child-join paths. I think we might consider that in future, though.
>
> You generally want to put the Gather node as high up in the plan tree
> as possible. I think the only case in which this is beneficial is if
> you can't put the Gather or Gather Merge node above the Append because
> only some of the children are parallel-safe. In that case, a separate
> Gather per child can be better than no parallelism at all. It's a
> rare case, but it can happen. Actually, I thought we had code for this
> already: see the end of apply_scanjoin_target_to_paths().
Agreed. Thanks for the explanation!
(I think that at least currently, there is no need for the Gather and
GatherMerge cases in reparameterize_path_by_child, but I don't object to
keeping those as-is there.)
Best regards,
Etsuro Fujita
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To: pgsql-hackers@postgresql.org
Cc: fujita.etsuro@lab.ntt.co.jp, robertmhaas@gmail.com, ashutosh.bapat@enterprisedb.com, michael@paquier.xyz, sawada.mshk@gmail.com
Subject: Re: print_path is missing GatherMerge and CustomScan support
In-Reply-To: <5B5ABE70.40606@lab.ntt.co.jp>
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