On Tue, Jun 25, 2019 at 9:53 AM James Coleman <jtc...@gmail.com> wrote: > Given the patch contents I don't see any obviously reason why either > of those possibilities (calling comparetup_heap less often, or it's > cheaper) are likely. Is that something we should investigate further? > Or is it just a nice happy accident that we should ignore since it's > dataset specific?
Have you actually confirmed that comparetup_heap() is called less often, by instrumenting the number of individual calls specifically? If there has been a reduction in calls to comparetup_heap(), then that's weird, and needs to be explained. If it turns out that it isn't actually called less often, then I would suggest that the speedup might be related to memory fragmentation, which often matters a lot within tuplesort.c. (This is why external sort merging now uses big buffers, and double buffering.) -- Peter Geoghegan
