Re: Extended statistics improvement: multi-column MCV missing values

Hi Chengpeng, thanks for the review!

On 26 Jul 2026 at 5:16, Chengpeng Yan (<chengpeng_yan(at)outlook(dot)com>) wrote:

> For patch 0001, apart from Ilia's comments, I only have one minor issue
> with a comment in `mcv_can_cap()`:
>
> Expressions are not supported, they can match multiple rows.
>
> is not quite accurate. Matching multiple rows is not specific to
> expressions; ordinary equality clauses may do so as well when they do
> not fully constrain all dimensions. Expressions are excluded here
> instead because the current full-coverage check only considers columns.
>
> Other than that, patch 0001 looks good to me.

I've addressed the comments in the attached v7 patch.

Regarding patch 0002, the formula seems reasonable as an estimate, but I
> am not convinced that it can serve as an upper bound. Some non-MCV
> combinations may be more frequent than the average, so using it as a cap
> does not seem justified. It may be more or less accurate than the
> existing per-column estimate depending on the data distribution.

Fair point, an average over the non-MCV combinations is not a true
upper bound, it's an estimation. I've been looking at how to reframe it
(it is essentially the same estimate var_eq_const() applies for single
columns on an MCV miss), but I think that discussion deserves its own
time (with a proper math analysis) and shouldn't hold back the rest.

So I'd propose we set 0002 aside for now and proceed with 0001, whose
reasoning is not affected by this: the least common MCV item is a
valid bound for any combination not in the list. I'll come back to
the ndistinct part in a follow-up.

Best regards,
Enrique.