Re: Unwanted expression simplification in PG12b2

Hi,

On Wed, Jul 17, 2019 at 11:58 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> =?UTF-8?Q?Darafei_=22Kom=D1=8Fpa=22_Praliaskouski?= <me(at)komzpa(dot)net>
> writes:
> > Many thanks for the parallel improvements in Postgres 12. Here is one of
> > cases where a costy function gets moved from a parallel worker into main
> > one, rendering spatial processing single core once again on some queries.
> > Perhaps an assumption "expressions should be mashed together as much as
> > possible" should be reviewed and something along "biggest part of
> > expression should be pushed down into parallel worker"?
>
> I don't see anything in your test case that proves what you think it does.
> The expensive calculation *is* being done in the worker in the first
> example. It's not real clear to me why the first example is only choosing
> to use one worker rather than 3, but probably with a larger test case
> (ie bigger table) that decision would change.
>

Indeed, it seems I failed to minimize my example.

Here is the actual one, on 90GB table with 16M rows:
https://gist.github.com/Komzpa/8d5b9008ad60f9ccc62423c256e78b4c

I can share the table on request if needed, but hope that plan may be
enough.

--
Darafei Praliaskouski
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