On Wed, Mar 9, 2016 at 5:47 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Alexander Korotkov <a.korot...@postgrespro.ru> writes: > > I have a question about Sort path. AFAICS this question wasn't mentioned > in > > the upthread discussion. > > We're producing Sort plans in two ways: from explicit Sort paths, and > from > > other paths which implicitly assumes sorting (like MergeJoin path). > > Would it be better to produce Sort plan only from explicit Sort path? > Thus, > > MergeJoin path would add explicit children Sort paths. That would be more > > unified way. > > Meh. I think the only real effect that would have is to make it slower to > build MergeJoin paths (and in a typical join problem, we build a lot of > those). The entire point of the Path/Plan distinction is to postpone > until createplan.c any work that's not necessary to arrive at a cost > estimate. So the way MergeJoinPath works now seems fine to me. > > > I ask about this from point of view of my "Partial Sort" patch. The > absence > > of implicit sorts would help to make this patch more simple and clean. > > There are other implicit sorts besides those. Admittedly, the efficiency > argument for not making the sort explicit in UniquePath or MergeAppendPath > is a lot weaker than it is for MergeJoin, just because those are less > common path types. But getting rid of the behavior entirely would be > a lot of work, and I'm not convinced it'd be an improvement. >
Thank you for the explanation. I'll try to rebase "Partial Sort" leaving this situation as is. ------ Alexander Korotkov Postgres Professional: http://www.postgrespro.com The Russian Postgres Company
