Em qui., 15 de jul. de 2021 às 11:19, David Rowley <dgrowleyml@gmail.com> escreveu:
On Fri, 16 Jul 2021 at 01:44, James Coleman <jtc331@gmail.com> wrote: > > On Wed, Jul 14, 2021 at 9:22 PM David Rowley <dgrowleyml@gmail.com> wrote: > > > > On Thu, 15 Jul 2021 at 12:30, Ranier Vilela <ranier.vf@gmail.com> wrote: > > > > > > Em qua., 14 de jul. de 2021 às 21:21, David Rowley <dgrowleyml@gmail.com> escreveu: > > >> But, in v8 there is no additional branch, so no branch to mispredict. > > >> I don't really see how your explanation fits. > > > > > > In v8 the branch occurs at : > > > + if (ExecGetResultType(outerPlanState(sortstate))->natts == 1) > > > > You do know that branch is in a function that's only executed once > > during executor initialization, right? > > This is why I have a hard time believing there's a "real" change here > and not the result of either noise or something not really > controllable like executable layout changing.
Yeah, I think we likely are at the level where layout changes in the compiled code are going to make things hard to measure. I just want to make sure we're not going to end up with some regression that's actual and not random depending on layout changes of unrelated code. I think a branch that's taken consistently *should* be predicted correctly each time.
Anyway, I think all the comparisons with v7b can safely be ignored. As Ronan pointed out, v7b has some issues due to it not recording the sort method in the executor state that leads to it forgetting which method it used once we start pulling tuples from it. The reproductions of that are it calling tuplesort_gettupleslot() from the 2nd tuple onwards regardless of if we've done a datum or tuple sort.
Sorry for insisting on this.
Assuming v7b is doing it the wrong way, which I still don't think it is. Why is it still faster than v6 and v8?
