| From: | "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Richard Guo <guofenglinux(at)gmail(dot)com>, Jian Guo <gjian(at)vmware(dot)com>, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Hans Buschmann <buschmann(at)nidsa(dot)net>, "pgsql-hackers(at)lists(dot)postgresql(dot)org" <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Zhenghua Lyu <zlyu(at)vmware(dot)com> |
| Subject: | Re: Wrong rows estimations with joins of CTEs slows queries by more than factor 500 |
| Date: | 2023-11-17 03:45:44 |
| Message-ID: | CAKFQuwa-pA960B=Ev=6SSqzPv2Pu5BXgZC=J9wYfRB3-MJ1faQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thursday, November 16, 2023, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> That line of argument also leads to the conclusion that it'd be
> okay to expose info about the ordering of the CTE result to the
> upper planner. This patch doesn't do that, and I'm not sufficiently
> excited about the issue to go write some code. But if someone else
> does, I think we shouldn't exclude doing it on the grounds of wanting
> to preserve an optimization fence. The fence is sort of one-way
> in this line of thinking: information can propagate up to the outer
> planner level, but not down into the CTE plan.
>
This is indeed my understanding of what materialized means. Basically, the
CTE is done first and in isolation; but any knowledge of its result shape
can be used when referencing it.
David J.
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