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:
> I'm not sure how to fix this, but it seems estimate_num_groups() needs
> to do things differently. And I agree looking for the minimum ndistinct
> seems like the right approach. but doesn't estimate_num_groups()
> supposed to already do that? The comment says:
I've rewritten the code in the previous email. It looks like we can try
to rewrite estimate_num_groups to do it more effectively, but I haven't
done it yet.
Regarding the tiny change in the cost, my initial reason was to teach
cost_sort to differ sort orderings: begin by considering the number of
columns in the cost estimation and then consider the distinct estimation
of the first column.
BTW, it was triggered by user reports, where a slight change in the
balance between MergeAppend/GatherMerge/Sort/IncrementalSort (or columns
order) could give significant profit. Especially when grouping millions
of rows in 2-4 bytea columns.

--
regards, Andrei Lepikhov