Re: Incremental Sort Cost Estimation Instability

On 12/9/2024 16:57, Tomas Vondra wrote:
> On 9/12/24 12:12, David Rowley wrote:
>> On Thu, 12 Sept 2024 at 21:51, Andrei Lepikhov <lepihov(at)gmail(dot)com> wrote:
> but doesn't estimate_num_groups()
> supposed to already do that? The comment says:
>
> * 3. If the list contains Vars of different relations that are known equal
> * due to equivalence classes, then drop all but one of the Vars from each
> * known-equal set, keeping the one with smallest estimated # of values
> * (since the extra values of the others can't appear in joined rows).
> * Note the reason we only consider Vars of different relations is that
> * if we considered ones of the same rel, we'd be double-counting the
> * restriction selectivity of the equality in the next step.
>
> I haven't debugged this, but how come this doesn't do the trick?
I've got your point now.
Unfortunately, this comment says that estimate_num_groups removes
duplicates from the list of grouping expressions (see
exprs_known_equal). But it doesn't discover em_members to find the
most-fitted clause for each grouping position.

--
regards, Andrei Lepikhov