Re: Early WIP/PoC for inlining CTEs

From: Andreas Karlsson <andreas(at)proxel(dot)se>
To: David Fetter <david(at)fetter(dot)org>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
Cc: Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Early WIP/PoC for inlining CTEs
Date: 2018-10-01 12:55:12
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On 07/27/2018 10:10 AM, David Fetter wrote:
> On Fri, Jul 27, 2018 at 02:55:26PM +1200, Thomas Munro wrote:
>> On Thu, Jul 26, 2018 at 7:14 AM, David Fetter <david(at)fetter(dot)org> wrote:
>>> Please find attached the next version, which passes 'make check'.
>> ... but not 'make check-world' (contrib/postgres_fdw's EXPLAIN is different).
> Please find attached a patch that does.
> It doesn't always pass make installcheck-world, but I need to sleep
> rather than investigate that at the moment.

I took a quick look at this patch and I have a couple of comments.

1) Is it really safe, for backwards compatibility reasons, to inline
when there are volatile functions? I imagine that it is possible that
there are people who rely on that volatile functions inside a CTE are
always run.

Imagine this case:

WITH cte AS (
SELECT x, volatile_f(x) FROM tab ORDER BY x

It could change behavior if volatile_f() has side effects and we inline
the CTE. Is backwards compatibility for cases like this worth
preserving? People can get the old behavior by adding OFFSET 0 or
MATERIALIZED, but existing code would break.

2) The code in inline_cte_walker() is quite finicky. It looks correct to
me but it is hard to follow and some simple bug could easily be hiding
in there. I wonder if this code could be rewritten in some way to make
it easier to follow.

3) Can you recall what the failing test was because I have so far not
managed to reproduce it?


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