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

On Tue, 10 Nov 2020 at 12:49, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Alvaro Herrera <alvherre@alvh.no-ip.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.