Re: Parallel Plans and Cost of non-filter functions

From: Paul Ramsey <pramsey(at)cleverelephant(dot)ca>
To: Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Parallel Plans and Cost of non-filter functions
Date: 2017-11-03 22:56:24
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Just clarifying myself a little, since I made a dumb error partway through.

On Thu, Nov 2, 2017 at 10:09 AM, Paul Ramsey <pramsey(at)cleverelephant(dot)ca>

> I'm working on a custom aggregate, that generates a serialized data
> format. The preparation of the geometry before being formatted is pretty
> intense, so it is probably a good thing for that work to be done in
> parallel, in partial aggregates. Here's an example SQL call:
> EXPLAIN analyze
> SELECT length(ST_AsMVT(a)) FROM (
> SELECT ST_AsMVTGeom(p.geom, ::geometry_literal, 4096, 0, true), gid,
> fed_num
> FROM pts_10 p
> WHERE p.geom && ::geometry_literal
> AND p.geom IS NOT NULL
> ) a;
> The ST_AsMVTGeom() function can be comically expensive, it's really good
> when it's in partial aggregates. But the cost of the function seems to be
> ignored.
> (First note that, in order to consistently get parallel plans I have to
> brutally suppress parallel_tuple_cost, as described here http://blog.
> Whether I get a parallel aggregate seems entirely determined by the number
> of rows, not the cost of preparing those rows.

This is true, as far as I can tell and unfortunate. Feeding tables with
100ks of rows, I get parallel plans, feeding 10ks of rows, never do, no
matter how costly the work going on within. That's true of changing costs
on the subquery select list, and on the aggregate transfn.

> When changing the number of rows in the subquery, with a LIMIT, I can
> change from a seq scan to a paralllel seq scan and finally to a parallel
> aggregate, as the number of rows goes up.

I see now that as soon as I brought the LIMIT in, the plans had to go
sequential, just due to the nature of a LIMIT in a subquery. Ignore the
below, sorry.

> An odd effect: when I have enough rows to get a paralllel seq scan, I get
> flip it back to a seq scan, by *increasing* the cost of ST_AsMVTGeom. That
> seems odd and backwards.
> Is there anywhere a guide or rough description to how costs are used in
> determining parallel plans? The empirical approach starts to wear one down
> after a while :)
> P.

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