Horton

Re: Early WIP/PoC for inlining CTEs

Andreas Karlsson <andreas@proxel.se>

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From: Andreas Karlsson <andreas@proxel.se>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Thomas Munro <thomas.munro@enterprisedb.com>, Andrew Gierth <andrew@tao11.riddles.org.uk>, Tom Lane <tgl@sss.pgh.pa.us>, David Fetter <david@fetter.org>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2019-01-21T13:40:34Z
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 1/18/19 9:34 PM, Robert Haas wrote:
> On Thu, Jan 17, 2019 at 10:48 AM Andreas Karlsson <andreas@proxel.se> wrote:
>> On 1/11/19 8:10 PM, Robert Haas wrote:
>>> WITH cte_name [[NOT] MATERIALIZED] AS (query) main_query...
>>
>> Hm, when would one want "NOT MATERIALIZED"? I am not sure I see the
>> usefulness of forcing inlining other than if we by default do not inline
>> when a CTE is referenced multiple times.
> 
> When the planner materializes it, but the performance of the resulting
> plan therefore sucks, I suppose.
> 
> I don't feel super-strongly about this, and Tom is right that there
> may be cases where materialization is just not practical due to
> implementation restrictions.  But it's not crazy to imagine that
> inlining a multiply-referenced CTE might create opportunities for
> optimization at each of those places, perhaps not the same ones in
> each case, whereas materializing it results in doing extra work.

I see.

I have a minor biksheddish question about the syntax.

You proposed:

WITH cte_name [[NOT] MATERIALIZED] AS (query) main_query

While Andrew proposed:

WITH cte_name AS [[NOT] MATERIALIZED] (query) main_query

Do people have any preference between these two?

Andreas