From: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>
To: Jeff Janes <jeff.janes@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andrew Gierth <andrew@tao11.riddles.org.uk>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Problems with plan estimates in postgres_fdw
Date: Fri, 22 Feb 2019 17:17:24 +0900
Message-ID: <5C6FB014.9040403@lab.ntt.co.jp> (raw)
In-Reply-To: <CAMkU=1z1Ez7fb_P_0Bc1040npE5fCOnu0M1DFyOzCp=e=rBJCw@mail.gmail.com>
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<5BB739CD.6040002@lab.ntt.co.jp>
<5BBC4140.50403@lab.ntt.co.jp>
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<5C518631.5080401@lab.ntt.co.jp>
<CAMkU=1z1Ez7fb_P_0Bc1040npE5fCOnu0M1DFyOzCp=e=rBJCw@mail.gmail.com>
Hi Jeff,
(2019/02/21 6:19), Jeff Janes wrote:
> On Wed, Jan 30, 2019 at 6:12 AM Etsuro Fujita
> <fujita.etsuro@lab.ntt.co.jp <mailto:fujita.etsuro@lab.ntt.co.jp>> wrote:
>
> (2018/12/28 15:50), Etsuro Fujita wrote:
> > Attached is a new version of the
> > patch.
>
> Here is an updated version of the patch set. Changes are:
>
> * In the previous version, LIMIT without OFFSET was not performed
> remotely as the costs was calculated the same as the costs of
> performing
> it locally. (Actually, such LIMIT was performed remotely in a case in
> the postgres_fdw regression test, but that was due to a bug :(.) I
> think we should prefer performing such LIMIT remotely as that avoids
> extra row fetches from the remote that performing it locally might
> cause, so I tweaked the costs estimated in
> estimate_path_cost_size(), to
> ensure that we'll prefer performing such LIMIT remotely.
> With your tweaks, I'm still not seeing the ORDER-less LIMIT get pushed
> down when using use_remote_estimate in a simple test case, either with
> this set of patches, nor in the V4 set. However, without
> use_remote_estimate, the LIMIT is now getting pushed with these patches
> when it does not in HEAD.
Good catch! I think the root cause of that is: when using
use_remote_estimate, remote estimates obtained through remote EXPLAIN
are rounded off to two decimal places, which I completely overlooked.
Will fix. I think I can post a new version early next week.
> See attached test case, to be run in new database named 'foo'.
Thanks for the review and the test case!
Best regards,
Etsuro Fujita
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To: pgsql-hackers@postgresql.org
Cc: fujita.etsuro@lab.ntt.co.jp, jeff.janes@gmail.com, tgl@sss.pgh.pa.us, andrew@tao11.riddles.org.uk
Subject: Re: Problems with plan estimates in postgres_fdw
In-Reply-To: <5C6FB014.9040403@lab.ntt.co.jp>
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