Re: Row estimates for empty tables

po 24. 8. 2020 v 21:43 odesílatel Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
napsal:

>
>
> 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
>

I think it can work. It is a good enough solution for people who need a
different behaviour with minimal impact on people who don't need a change.

Regards

Pavel

>
> Regards
>
> Pavel
>
>>
>> regards, tom lane
>>
>>