Thread

Re: Is there a way to identify a plan generated by GECO?

Nikolay Samokhvalov <nik@postgres.ai> — 2025-07-18T01:43:56Z

On Thu, Jul 17, 2025 at 18:11 Jerry Brenner <jbrenner@guidewire.com> wrote:

> We are on Postgres 15.5 (Aurora)  and capturing query plans via
> auto_explain.  We are seeing a large number of query plans for 2 queries
> that have 12 tables.  Every fast (or "fast enough") plan has a left deep
> tree and every slow plan has a bushy tree.  Is there a way to determine if
> a plan was generated by GECO?
>
> We have from_collapse_limit, join_collapse_limit and geqo_threshold all
> set to 12.  (There is a COUNT(*) above derived table - could that be
> somehow affecting this?)
>
> I've manually explained plans and haven't seen the problem, but then it
> turns up the next day (with the same parameter values) with multiple
> execution plans.
>

I'm not aware of ability to see if grow was involved, but with Aurora,
should be able can provision a thin (CoW) clone with PITR to a specific
point when you suspect the plan in question was used ("slow") -- and study
the planner behavior in detail, experimenting and adjusting planner
parameters.

Nik

>