From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andrew Gierth <andrew@tao11.riddles.org.uk>
cc: Andreas Karlsson <andreas@proxel.se>, David Fetter <david@fetter.org>,
        Thomas Munro <thomas.munro@enterprisedb.com>,
        Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Early WIP/PoC for inlining CTEs
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Comments: In-reply-to Andrew Gierth <andrew@tao11.riddles.org.uk>
	message dated "Fri, 16 Nov 2018 23:04:38 +0000"
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Date: Fri, 16 Nov 2018 18:24:51 -0500
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Andrew Gierth <andrew@tao11.riddles.org.uk> writes:
> "Tom" == Tom Lane <tgl@sss.pgh.pa.us> writes:
>  Tom> * I have no faith in the idea that we can skip doing a copyObject
>  Tom> on the inlined subquery, except maybe in the case where we know
>  Tom> there's exactly one reference.

> The code doesn't do a copyObject on the query if there are no level
> changes because everywhere else just does another copyObject on it as
> the first processing step (cf. set_subquery_pathlist and the various
> functions called from pull_up_subqueries).

Perhaps it's safe today, but is that likely to remain true?  We've had
enough pain in the past from multiply-linked parse subtrees that I am
not eager to introduce another case, especially not here where there's
absolutely no evidence that it'd provide a meaningful performance
improvement.

>  Tom> * Speaking of the comments, I'm not convinced that view rewrite is
>  Tom> a good comparison point; I think this is more like subquery
>  Tom> pullup.

> It's not really like subquery pullup because we actually do go on to do
> subquery pullup _after_ inlining CTEs.

Subquery pullup can happen across multiple levels, too.

			regards, tom lane