pg.ddx.io  pgsql-hackers@postgresql.org mailing list archive  

help / color / mirror / Atom feed
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: David Rowley <david.rowley@2ndquadrant.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Simon Riggs <simon@2ndquadrant.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Performance improvement for joins where outer side is unique
Date: Sat, 12 Mar 2016 12:32:31 -0500
Message-ID: <29063.1457803951@sss.pgh.pa.us> (raw)
In-Reply-To: <CA+TgmoYJc+hBsvCQiPpuHM6sCRc1NGq5FWO10yeJtSeoH4ji8w@mail.gmail.com>
References: <20150320.121140.148610378.horiguchi.kyotaro@lab.ntt.co.jp>
	<CAPpHfdviphQFjk-gaGZGGxLAyt=YuVdJLnntsxOKnU296s9u-Q@mail.gmail.com>
	<CAKJS1f-=1y0-f5FFV2_r7LtS+CL2Lw-nhJjXB3O_0FmBqnK=3A@mail.gmail.com>
	<27152324.eFKJb4Khb5@dinodell>
	<8608754a32d1325c2f83ba8b1df71585@xs4all.nl>
	<55DA1F9F.5040204@2ndquadrant.com>
	<CAKJS1f-9UTo6fA+h8rojd30CujSKscZamLn5YnKw5+ASbV0Wwg@mail.gmail.com>
	<5085.1440375543@sss.pgh.pa.us>
	<CAKJS1f_8hrzTDCu0pwERWj_dLZu-6DYFQJAtNT+vM6UeOn0kHw@mail.gmail.com>
	<8907.1440383377@sss.pgh.pa.us>
	<CAKJS1f8JCt15ngEP8sZO-JYYT8mS6HUP_CT0T6S_0ogk1kQt+Q@mail.gmail.com>
	<CAKJS1f_4hm3qwKeHYKftd+BQuYBgdNR+rk+XP_JW9EY-ALHZbQ@mail.gmail.com>
	<56718B15.6060507@2ndquadrant.com>
	<CAKJS1f_i7dGh9ytc0DuaX3JJaN7Gp9AGXNbdtWjj0PkPzNzUeg@mail.gmail.com>
	<5671FF0D.2010403@2ndquadrant.com>
	<CANP8+j+vxfEf3dqhnzKAnDopnnUM-Zjb90gNT=haEQNQePj9Mw@mail.gmail.com>
	<CAKJS1f8MpcL4_oS+n-vmnisd_8OMuZR2vyLETOF7ABSLk_iNZg@mail.gmail.com>
	<56A25A72.2090505@2ndquadrant.com>
	<24304.145748278! 0@sss.pgh.pa.us>
	<CAKJS1f-qN8NVT456XT6-NW+Vc_n0-jfCVtaQio0JoOWzP6F2ig@mail.gmail.com>
	<13653.1457731978@sss.pgh.pa.us>
	<CA+TgmoYJc+hBsvCQiPpuHM6sCRc1NGq5FWO10yeJtSeoH4ji8w@mail.gmail.com>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>

Robert Haas <robertmhaas@gmail.com> writes:
> The new join pushdown code in postgres_fdw does not grok SEMI and ANTI
> joins because there is no straightforward way of reducing those back
> to SQL.  They can originate in multiple ways and not all of those can
> be represented easily.  I think it would be nice to do something to
> fix this.  For example, if a LEFT join WHERE outer_column IS NULL
> turns into an ANTI join, it would be nice if that were marked in some
> way so that postgres_fdw could conveniently emit it in the original
> form.

> Maybe the people who have been working on that patch just haven't been
> creative enough in thinking about how to solve this problem, but it
> makes me greet the idea of more join types that don't map directly
> back to SQL with somewhat mixed feelings.

I can't summon a whole lot of sympathy for that objection.  These cases
won't arise with postgres_fdw as it stands because we'd never be able to
prove uniqueness on a foreign table.  When and if someone tries to improve
that, we can think about how the whole thing ought to map to FDWs.

Having said that, your point does add a bit of weight to David's
suggestion of inventing two new join-type codes rather than overloading
JOIN_SEMI.  I'd still be a bit inclined to display JOIN_INNER_UNIQUE or
whatever we call it as a "Semi" join in EXPLAIN, though, just to minimize
the amount of newness there.

			regards, tom lane


-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers



view thread (131+ messages)  latest in thread

Message-ID: <29063.1457803951@sss.pgh.pa.us>
Permalink:  ../29063.1457803951@sss.pgh.pa.us/
Also on:    postgresql.org/message-id/29063.1457803951@sss.pgh.pa.us

 · 

reply
Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: pgsql-hackers@postgresql.org
  Cc: tgl@sss.pgh.pa.us, robertmhaas@gmail.com, david.rowley@2ndquadrant.com, tomas.vondra@2ndquadrant.com, simon@2ndquadrant.com
  Subject: Re: Performance improvement for joins where outer side is unique
  In-Reply-To: <29063.1457803951@sss.pgh.pa.us>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox