Re: MergeAppend could consider sorting cheapest child path

On 4/25/25 17:13, Andrei Lepikhov wrote:
> On 4/25/25 11:16, Alexander Pyhalov wrote:
>> Usually, sorted cheapest_total_path will be cheaper than sorted
>> fractional/startup path at least by startup cost (as after sorting it
>> includes total_cost of input path). But we ignore this case when
>> selecting cheapest_startup and cheapest_fractional subpaths. As result
>> selected cheapest_startup and cheapest_fractional can be not cheapest
>> for startup or selecting a fraction of rows.
> I don't know what you mean by that. The cheapest_total_path is
> considered when we chose optimal cheapest_total path. The same works for
> the fractional path - get_cheapest_fractional_path gives us the most
> optimal fractional path and probes cheapest_total_path too.
> As above, not sure about min-startup case for now. I can imagine
> MergeAppend over sophisticated subquery: non-sorted includes highly
> parameterised JOINs and the alternative (with pathkeys) includes
> HashJoin, drastically increasing startup cost. It is only a theory, of
> course. So, lets discover how min-startup works.
After a second thought I have caught your idea. I agree that for a
fractional path it have no sense to choose any other path except a
cheapest total one.
There are the modified patch in the attachment.

Also, to be more objective, I propose to use examples in argumentation -
something like in attached test2.sql script.

--
regards, Andrei Lepikhov