Re: Row estimates for empty tables

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Christophe Pettus <xof(at)thebuild(dot)com>, David Rowley <dgrowleyml(at)gmail(dot)com>
Subject: Re: Row estimates for empty tables
Date: 2020-08-23 21:08:16
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Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> I am sending a patch that is years used in GoodData.

I'm quite unexcited about that. I'd be the first to agree that the
ten-pages estimate is a hack, but it's not an improvement to ask users
to think of a better value ... especially not as a one-size-fits-
all-relations GUC setting.

I did have an idea that I think is better than my previous one:
rather than lying about the value of relpages, let's represent the
case where we don't know the tuple density by setting reltuples = -1
initially. This leads to a patch that's a good bit more invasive than
the quick-hack solution, but I think it's a lot cleaner on the whole.

A possible objection is that this changes the FDW API slightly, as
GetForeignRelSize callbacks now need to deal with rel->tuples possibly
being -1. We could avoid an API break if we made plancat.c clamp
that value to zero; but then FDWs still couldn't tell the difference
between the "empty" and "never analyzed" cases, and I think this is
just as much of an issue for them as for the core code.

I'll add this to the upcoming CF.

regards, tom lane

Attachment Content-Type Size
use-minus-one-for-unknown-reltuples-1.patch text/x-diff 19.5 KB

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