Re: Propagate stadistinct through GROUP BY/DISTINCT in subqueries and CTEs

On Mon, Apr 13, 2026 at 10:18 AM Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
> For DISTINCT or GROUP BY key columns that are simple Vars, I think we
> can propagate stadistinct from the base table, because the set of
> distinct values is preserved after grouping. MCV frequencies,
> histograms, and correlation data are not valid since GROUP BY and
> DISTINCT change the frequency distribution, but with stadistinct
> alone, callers like var_eq_const() can use a 1/ndistinct estimate
> rather than 1/DEFAULT_NUM_DISTINCT.
>
> Attached is a patch to do this.

Upon self-review, I realized that stanullfrac wasn't adjusted. Since
grouping collapses NULL values, we should account for that here.

Attached is v2 addressing this. For a single grouping key, at most
one NULL group remains, so we set it to 1 / (ndistinct + 1). With
multiple grouping keys, the surviving NULL count is underdetermined,
so we approximate it as zero; NULLs collapse far more aggressively
than non-NULLs, so the real fraction is well below the base table's,
and erring low keeps estimates on the hash-join-favoring side.

- Richard