| From: | Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> |
|---|---|
| To: | David Rowley <dgrowleyml(at)gmail(dot)com> |
| Cc: | Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andres Freund <andres(at)anarazel(dot)de>, PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Hybrid Hash/Nested Loop joins and caching results from subplans |
| Date: | 2020-11-09 23:15:30 |
| Message-ID: | 20201109231530.GA14767@alvherre.pgsql |
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| Lists: | pgsql-hackers |
On 2020-Nov-10, David Rowley wrote:
> On Mon, 9 Nov 2020 at 16:29, Andy Fan <zhihui(dot)fan1213(at)gmail(dot)com> wrote:
> > However I believe v9
> > should be no worse than v8 all the time, Is there any theory to explain
> > your result?
>
> Nothing jumps out at me from looking at profiles. The only thing I
> noticed was the tuple deforming is more costly with v9. I'm not sure
> why.
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.
I don't know what is a good technique to test this hypothesis.
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