Subject: Re: updated hash functions for postgresql v1
From: Simon Riggs <simon@2ndquadrant.com>
To: Kenneth Marshall <ktm@rice.edu>
Cc: pgsql-patches@postgresql.org, twraney@comcast.net, neilc@samurai.com
In-Reply-To: <20071028180558.GY27320@it.is.rice.edu>
References: <20071027201557.GW27320@it.is.rice.edu>
	<1193592458.4242.713.camel@ebony.site>
	<20071028180558.GY27320@it.is.rice.edu>
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Date: Sun, 28 Oct 2007 20:06:58 +0000
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On Sun, 2007-10-28 at 13:05 -0500, Kenneth Marshall wrote:
> On Sun, Oct 28, 2007 at 05:27:38PM +0000, Simon Riggs wrote:
> > On Sat, 2007-10-27 at 15:15 -0500, Kenneth Marshall wrote:
> > > Its features include a better and faster hash function.
> > 
> > Looks very promising. Do you have any performance test results to show
> > it really is faster, when compiled into Postgres? Better probably needs
> > some definition also; in what way are the hash functions better?
> >  
> > -- 
> >   Simon Riggs
> >   2ndQuadrant  http://www.2ndQuadrant.com
> > 
> The new hash function is roughly twice as fast as the old function in
> terms of straight CPU time. It uses the same design as the current
> hash but provides code paths for aligned and unaligned access as well
> as separate mixing functions for different blocks in the hash run
> instead of having one general purpose block. I think the speed will
> not be an obvious win with smaller items, but will be very important
> when hashing larger items (up to 32kb).
> 
> Better in this case means that the new hash mixes more thoroughly
> which results in less collisions and more even bucket distribution.
> There is also a 64-bit varient which is still faster since it can
> take advantage of the 64-bit processor instruction set.

Ken, I was really looking for some tests that show both of the above
were true. We've had some trouble proving the claims of other algorithms
before, so I'm less inclined to take those things at face value.

I'd suggest tests with Integers, BigInts, UUID, CHAR(20) and CHAR(100).
Others may have different concerns.

-- 
  Simon Riggs
  2ndQuadrant  http://www.2ndQuadrant.com