Julien Rouhaud <rjuju123@gmail.com> writes:
> On Fri, Mar 8, 2019 at 9:15 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I think you should remove all that
>> and restrict this optimization to the case where all the subpaths are
>> natively ordered --- if we have to insert Sorts, it's hardly going to move
>> the needle to worry about simplifying the parent MergeAppend to Append.
> This can be a huge win for queries of the form "ORDER BY partkey LIMIT
> x". Even if the first subpath(s) aren't natively ordered, not all of
> the sorts should actually be performed.
[ shrug... ] We've got no realistic chance of estimating such situations
properly, so I'd have no confidence in a plan choice based on such a
thing. Nor do I believe that this case is all that important.
regards, tom lane