Re: Row estimates for empty tables

ne 23. 8. 2020 v 23:08 odesílatel Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> napsal:

> 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.
>

This patch is just a workaround that works well 10 years (but for one
special use case) - nothing more. Without this patch that application
cannot work ever.

> 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.
>

I'll check it

Regards

Pavel

>
> regards, tom lane
>
>