Re: Hybrid Hash/Nested Loop joins and caching results from subplans

On Tue, Nov 10, 2020 at 7:55 AM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:

> On Tue, 10 Nov 2020 at 12:49, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >
> > Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> writes:
> > > Are you taking into account the possibility that generated machine code
> > > is a small percent slower out of mere bad luck? I remember someone
> > > suggesting that they can make code 2% faster or so by inserting random
> > > no-op instructions in the binary, or something like that. So if the
> > > difference between v8 and v9 is that small, then it might be due to
> this
> > > kind of effect.
> >
> > Yeah. I believe what this arises from is good or bad luck about relevant
> > tight loops falling within or across cache lines, and that sort of thing.
> > We've definitely seen performance changes up to a couple percent with
> > no apparent change to the relevant code.
>
> I do happen to prefer having the separate Result Cache node (v8), so
> from my point of view, even if the performance was equal, I'd rather
> have v8. I understand that some others feel different though.
>
>
While I have interest about what caused the tiny difference, I admit that
what direction
this patch should go is more important. Not sure if anyone is convinced
that
v8 and v9 have a similar performance. The current data show it is similar.
I want to
profile/read code more, but I don't know what part I should pay attention
to. So I think
any hints on why v9 should be better at a noticeable level in theory
should be very
helpful. After that, I'd like to read the code or profile more carefully.

--
Best Regards
Andy Fan