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From: Antonin Houska <ah@cybertec.at>
To: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andrew Gierth <andrew@tao11.riddles.org.uk>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Problems with plan estimates in postgres_fdw
Date: Fri, 08 Mar 2019 17:25:49 +0100
Message-ID: <8642.1552062349@localhost> (raw)
In-Reply-To: <5C7E4127.2030802@lab.ntt.co.jp>
References: <87pnz1aby9.fsf@news-spur.riddles.org.uk>
	<1175.1533220908@sss.pgh.pa.us>
	<5BB739CD.6040002@lab.ntt.co.jp>
	<5BBC4140.50403@lab.ntt.co.jp>
	<5C17A00E.5000005@lab.ntt.co.jp>
	<5C232F39.9060509@lab.ntt.co.jp>
	<5C25C7B5.2010802@lab.ntt.co.jp>
	<5C518631.5080401@lab.ntt.co.jp>
	<10994.1549559088@localhost>
	<5C5D778F.3080501@lab.ntt.co.jp>
	<5C6D474F.5010703@lab.ntt.co.jp>
	<30939.1550848901@localhost>
	<5C73B7FB.2010309@lab.ntt.co.jp>
	<7520.1551439018@localhost>
	<5C7E4127.2030802@lab.ntt.co.jp>

Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp> wrote:

> (2019/03/01 20:16), Antonin Houska wrote:
> > Etsuro Fujita<fujita.etsuro@lab.ntt.co.jp>  wrote:
> 
> >> Conversely, it appears that add_foreign_ordered_paths() added by the patchset
> >> would generate such pre-sorted paths *redundantly* when the input_rel is the
> >> final scan/join relation.  Will look into that.
> >
> > Currently I have no idea how to check the plan created by FDW at the
> > UPPERREL_ORDERED stage, except for removing the sort from the
> > UPPERREL_GROUP_AGG stage as I proposed here:
> >
> > https://www.postgresql.org/message-id/11807.1549564431%40localhost
> 
> I don't understand your words "how to check the plan created by FDW". Could
> you elaborate on that a bit more?

I meant that I don't know how to verify that the plan that sends the ORDER BY
clause to the remote server was created at the UPPERREL_ORDERED and not at
UPPERREL_GROUP_AGG. However if the ORDER BY clause really should not be added
at the UPPERREL_GROUP_AGG stage and if we ensure that it no longer happens,
then mere presence of ORDER BY in the (remote) plan means that the
UPPERREL_ORDERED stage works fine.

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




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