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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Richard Guo <guofenglinux@gmail.com>
Cc: Jian Guo <gjian@vmware.com>
Cc: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Hans Buschmann <buschmann@nidsa.net>
Cc: pgsql-hackers@lists.postgresql.org <pgsql-hackers@lists.postgresql.org>
Cc: Zhenghua Lyu <zlyu@vmware.com>
Subject: Re: Wrong rows estimations with joins of CTEs slows queries by more than factor 500
Date: Mon, 20 Nov 2023 12:45:58 -0500
Message-ID: <1339607.1700502358@sss.pgh.pa.us> (raw)
In-Reply-To: <CAMbWs49gAHeEOn0rpdUUYXryaa60KZ8JKwk1aSERttY9caCYkA@mail.gmail.com>
References: <4504e67078d648cdac3651b2960da6e7@nidsa.net>
	<SN6PR05MB5199BB4BB8C982BB519B6660C417A@SN6PR05MB5199.namprd05.prod.outlook.com>
	<46a7d950-7ca7-8ab9-76dd-f0bc7611c609@enterprisedb.com>
	<SN6PR05MB5199C0095508555B7D8D0D23C41EA@SN6PR05MB5199.namprd05.prod.outlook.com>
	<891440.1699483495@sss.pgh.pa.us>
	<CAMbWs4-W_81Q3ff1Bk3NU_ChRzOJK0njAoJanW-kXnPF0wjd1Q@mail.gmail.com>
	<414317.1700158568@sss.pgh.pa.us>
	<CAMbWs48Z_PvveOeankGA69=V=fqogusRKGVPBUGfYC0zDo1WPA@mail.gmail.com>
	<482957.1700192299@sss.pgh.pa.us>
	<CAMbWs49gAHeEOn0rpdUUYXryaa60KZ8JKwk1aSERttY9caCYkA@mail.gmail.com>

Richard Guo <guofenglinux@gmail.com> writes:
> On Fri, Nov 17, 2023 at 11:38 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> That line of argument also leads to the conclusion that it'd be
>> okay to expose info about the ordering of the CTE result to the
>> upper planner.

> In the light of this conclusion, I had a go at propagating the pathkeys
> from CTEs up to the outer planner and came up with the attached.

Oh, nice!  I remembered we had code already to do this for regular
SubqueryScans, but I thought we'd need to do some refactoring to
apply it to CTEs.  I think you are right though that
convert_subquery_pathkeys can be used as-is.  Some thoughts:

* Do we really need to use make_tlist_from_pathtarget?  Why isn't
the tlist of the cteplan good enough (indeed, more so)?

* I don't love having this code assume that it knows how to find
the Path the cteplan was made from.  It'd be better to make
SS_process_ctes save that somewhere, maybe in a list paralleling
root->cte_plan_ids.

Alternatively: maybe it's time to do what the comments in
SS_process_ctes vaguely speculate about, and just save the Path
at that point, with construction of the plan left for createplan()?
That might be a lot of refactoring for not much gain, so not sure.

			regards, tom lane





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