| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Andreas Karlsson <andreas(at)proxel(dot)se> |
| Cc: | Andres Freund <andres(at)anarazel(dot)de>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, David Fetter <david(at)fetter(dot)org>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Early WIP/PoC for inlining CTEs |
| Date: | 2019-01-18 20:34:46 |
| Message-ID: | CA+TgmoapASa3RQi=DuSikPoPF4363czSD8KxbOxsfW5XmO3WpQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Jan 17, 2019 at 10:48 AM Andreas Karlsson <andreas(at)proxel(dot)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.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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