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Re: hash index improving v3

From: "Alex Hunsaker" <badalex(at)gmail(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Zdenek Kotala" <Zdenek(dot)Kotala(at)sun(dot)com>, "Xiao Meng" <mx(dot)cogito(at)gmail(dot)com>, pgsql-patches(at)postgresql(dot)org, "Jonah H(dot) Harris" <jonah(dot)harris(at)gmail(dot)com>
Subject: Re: hash index improving v3
Date: 2008-09-05 01:51:14
Message-ID: 34d269d40809041851r3330198ej6ccd0ae53e4260f9@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
On Thu, Sep 4, 2008 at 7:13 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> "Alex Hunsaker" <badalex(at)gmail(dot)com> writes:
>> Ok let me know if this is to naive of an approach or not hitting the
>> right cases you want tested.
>
> You have the unique-versus-not dimension, but I'm also wondering about
> narrow vs wide index keys (say about 8 bytes vs 50-100 or so).  In the
> former case we're not saving any index space by storing only the hash
> code, so these could be expected to have different performance
> behaviors.

Arg yes... I just read the last part of your mail in this thread.  I
think it was the one on -hackers that talked about narrow vs wide...
so I figured I would just try to do what the thread where you posted
the patch talked about namley the below:

>So my thinking right now is that we should just test this patch as-is.
>If it doesn't show really horrid performance when there are lots of
>hash key collisions, we should forget the store-both-things idea and
>just go with this.

So I thought, lets try to generate lots of hash collisions... obviosly
though using the same key wont do that... Not sure what I was thinking

> As for specifics of the suggested scripts:
>
> * might be better to do select count(*) not select 1, so that client
> communication is minimized

Yar.

> * check that the queries actually use the indexes (not sure that the
> proposed switch settings ensure this, not to mention you didn't create
> the indexes)

Well I was assuming I could just test the speed of a hash join...

> * make sure the pgbench transaction counts are large enough to ensure
> significant runtime
> * the specific table sizes suggested are surely not large enough

Ok

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