Re: [BUG?] estimate_hash_bucket_stats uses wrong ndistinct for avgfreq

"Joel Jacobson" <joel(at)compiler(dot)org> writes:
> On Sun, Mar 1, 2026, at 22:12, Tom Lane wrote:
>> Aside: you could argue that failing to consider stanullfrac is wrong,
>> and maybe it is. But the more I looked at this code the more
>> convinced I got that it was only partially accounting for nulls
>> anyway. That seems like perhaps something to look into later.

> How about adjusting estfract for the null fraction before clamping?

This reminds me of the unfinished business at [1]. We really ought
to make it true that nulls never get into the hash table before
we assume that's so in costing. One of the things I was thinking
was being overlooked is the possibility of lots of nulls bloating
whichever hash bucket they get put in --- but if they aren't put
into a bucket then it's not wrong to ignore them here.

(Strictly speaking, that's still not so with non-strict hash operators,
but those are so rare that I don't mind not accounting for them.)

regards, tom lane

[1] https://www.postgresql.org/message-id/flat/3061845(dot)1746486714(at)sss(dot)pgh(dot)pa(dot)us