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List:       pgsql-performance
Subject:    Re: [PERFORM] HASH
From:       Jeff Janes <jeff.janes () gmail ! com>
Date:       2015-11-05 11:24:24
Message-ID: CAMkU=1x=if-tO1CkoVPqLnWxz8nYtu4Gs3JBw5ceLxFgVwwfkg () mail ! gmail ! com
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On Thu, Nov 5, 2015 at 1:11 AM, Artem Tomyuk <admin@leboutique.com> wrote:
> Hi all.
>
> Is the speed of hash operations stands on the performance of CPU?

Yes, but the variation is probably not as much as the raw timing in
your example indicates.

> Below you can see part from output of explain analyze command
>
> Intel(R) Xeon(R) CPU           E7520  @ 1.87GHz
>
> "              ->  Hash  (cost=337389.43..337389.43 rows=3224443 width=34)
> (actual time=15046.382..15046.382 rows=3225191 loops=1)"
> "                    Buckets: 524288  Batches: 1  Memory Usage: 207874kB"

A lot of that time was probably spent reading the data off of disk so
that it could hash it.

You should turn track_io_timing on, run "explain (analyze, buffers)
..." and then show the entire explain output, or at least also show
the entries downstream of the Hash node.

Cheers,

Jeff


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