Re: [PATCH] Incremental sort (was: PoC: Partial sort)

On Tue, Jun 25, 2019 at 9:53 AM James Coleman <jtc331@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