Horton

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

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, David Rowley <dgrowleyml@gmail.com>, Andy Fan <zhihui.fan1213@gmail.com>, Andres Freund <andres@anarazel.de>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2020-11-09T23:55:38Z
Lists: pgsql-hackers 
On Mon, Nov 9, 2020 at 3:49 PM 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.

That was Andrew Gierth. And it was 5% IIRC.

In theory it should be possible to control for this using a tool like
stabilizer:

https://github.com/ccurtsinger/stabilizer

I am not aware of anybody having actually used the tool with Postgres,
though. It looks rather inconvenient.

-- 
Peter Geoghegan

Commits

  1. Add Result Cache executor node (take 2)

  2. Add Result Cache executor node

  3. Allow estimate_num_groups() to pass back further details about the estimation

  4. Allow users of simplehash.h to perform direct deletions

  5. Cache if PathTarget and RestrictInfos contain volatile functions

  6. Fix pull_varnos' miscomputation of relids set for a PlaceHolderVar.