From: Antonin Houska <ah@cybertec.at>
To: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>
cc: Tom Lane <tgl@sss.pgh.pa.us>,
    Andrew Gierth <andrew@tao11.riddles.org.uk>,
    pgsql-hackers@postgresql.org
Subject: Re: Problems with plan estimates in postgres_fdw
In-reply-to: <5C6D474F.5010703@lab.ntt.co.jp>
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Comments: In-reply-to Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>
   message dated "Wed, 20 Feb 2019 21:25:51 +0900."
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Date: Fri, 22 Feb 2019 16:21:41 +0100
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Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp> wrote:

> (2019/02/08 21:35), Etsuro Fujita wrote:

> > (2019/02/08 2:04), Antonin Houska wrote:

> >> * regression tests: I think test(s) should be added for queries that have
> >> ORDER BY clause but do not have GROUP BY (and also no LIMIT / OFFSET)
> >> clause. I haven't noticed such tests.
> >
> > Will do.
> 
> I noticed that such queries would be processed by what we already have for
> sort pushdown (ie, add_paths_with_pathkeys_for_rel()).  I might be missing
> something, though.

What about an ORDER BY expression that contains multiple Var nodes? For
example

SELECT * FROM foo ORDER BY x + y;

I think the base relation should not be able to generate paths with the
corresponding pathkeys, since its target should (besides PlaceHolderVars,
which should not appear in the plan of this simple query at all) only emit
individual Vars.

-- 
Antonin Houska
https://www.cybertec-postgresql.com