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.

It possibly is this issue.

Normally how I build up my confidence in which is faster is why just
rebasing on master as it advances and see if the winner ever changes.
The theory here is if one patch is consistently the fastest, then
there's more chance if there being a genuine reason for it.

So far I've only rebased v9 twice. Both times it was slower than v8.
Since the benchmarks are all scripted, it's simple enough to kick off
another round to see if anything has changed.

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.

David