Julien Rouhaud <rjuju...@gmail.com> writes: > On Fri, Mar 8, 2019 at 9:15 PM Tom Lane <t...@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