Horton

Re: Early WIP/PoC for inlining CTEs

Andreas Karlsson <andreas@proxel.se>

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From: Andreas Karlsson <andreas@proxel.se>
To: David Fetter <david@fetter.org>, Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Andrew Gierth <andrew@tao11.riddles.org.uk>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2018-10-01T12:55:12Z
Lists: pgsql-hackers

Commits

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  1. Prevent inlining of multiply-referenced CTEs with outer recursive refs.

  2. Allow user control of CTE materialization, and change the default behavior.

  3. Split QTW_EXAMINE_RTES flag into QTW_EXAMINE_RTES_BEFORE/_AFTER.

  4. document when PREPARE uses generic plans

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@fetter.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
)
SELECT * FROM cte LIMIT 10;

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?

Andreas