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From: Andrew Gierth <andrew@tao11.riddles.org.uk>
To: Andreas Karlsson <andreas@proxel.se>
Cc: David Fetter <david@fetter.org>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Early WIP/PoC for inlining CTEs
Date: Fri, 05 Oct 2018 01:40:05 +0100
Message-ID: <87y3bdckvu.fsf@news-spur.riddles.org.uk> (raw)
In-Reply-To: <5bad579d-7a99-74de-2311-b4f049da8b40@proxel.se>
References: <87sh48ffhb.fsf@news-spur.riddles.org.uk>
	<20180725054236.GB17411@fetter.org>
	<87bmavfjhy.fsf@news-spur.riddles.org.uk>
	<20180725191447.GE17411@fetter.org>
	<CAEepm=0MKV3MnR4Dn5RkXsK+RnmHEFitqiG4PcBhXtBK4JOoWw@mail.gmail.com>
	<20180727081058.GN17411@fetter.org>
	<dccb37dd-2f52-dbd7-a4c4-72a5853f4838@proxel.se>
	<e02af96a-3e25-5de4-09a4-a7ebf3452184@proxel.se>
	<20181003155720.GG25294@fetter.org>
	<5bad579d-7a99-74de-2311-b4f049da8b40@proxel.se>

>>>>> "Andreas" == Andreas Karlsson <andreas@proxel.se> writes:

 > On 10/03/2018 05:57 PM, David Fetter wrote:
 >> Is there any meaningful distinction between "inlining," by which I
 >> mean converting to a subquery, and predicate pushdown, which
 >> would happen at least for a first cut, at the rewrite stage?

Yes.

 Andreas> Sorry, but I do not think I understand your question. The
 Andreas> ability to push down predicates is just one of the potential
 Andreas> benefits from inlining.

Consider the difference between (in the absence of CTE inlining):

-- inline subquery with no optimization barrier (qual may be pushed down)
select * from (select x from y) s where x=1;

-- inline subquery with optimization barrier (qual not pushed down)
select * from (select x from y offset 0) s where x=1;

-- CTE with materialization
with s as (select x from y) select * from s where x=1;

-- 
Andrew (irc:RhodiumToad)




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  Cc: andrew@tao11.riddles.org.uk, andreas@proxel.se, david@fetter.org, thomas.munro@enterprisedb.com
  Subject: Re: Early WIP/PoC for inlining CTEs
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