Re: Improve UNION's output rowcount estimate

On Mon, Jun 22, 2026 at 9:39 AM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> I believe this should make the following code redundant, so shouldn't
> the patch remove it too?
>
> /*
> * Estimate the number of UNION output rows. In the case when only a
> * single UNION child remains, we can use estimate_num_groups() on
> * that child. We must be careful not to do this when that child is
> * the result of some other set operation as the targetlist will
> * contain Vars with varno==0, which estimate_num_groups() wouldn't
> * like.
> */
> if (list_length(cheapest.subpaths) == 1 &&
> first_path->parent->reloptkind != RELOPT_UPPER_REL)
> {
> dNumGroups = estimate_num_groups(root,
> first_path->pathtarget->exprs,
> first_path->rows,
> NULL,
> NULL);
> }
>
> Then you may as well pass dNumChildGroups directly to the path
> creation functions and get rid of your new "With multiple children,"
> comment.
>
> Aside from that, I don't see any issues.

Thanks for looking. You're right. build_setop_child_paths() already
computes each child's distinct estimate, so for a single surviving
child dNumChildGroups is exactly what that branch recomputed.

(And removing it can be a slight improvement, as the old branch ran
estimate_num_groups on the subquery-scan Vars, while
build_setop_child_paths uses the child's own rowcount when it has
GROUP BY/DISTINCT/aggs.)

Patch updated.

- Richard