Re: hash index improving v3

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Alex Hunsaker" <badalex(at)gmail(dot)com>
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 01:45:53
Message-ID: 11525.1220579153@sss.pgh.pa.us
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I wrote:
> You have the unique-versus-not dimension,

On second thought, actually not. What we want to look at is the penalty
for false matches due to *distinct* key values that happen to have the
same hash codes. Your test case for all-the-same is using all the same
key values, which means it'll hit the heap a lot, but none of those will
be wasted trips.

So what we need for testing is a few different key values that hash to
the same code. Not sure about an easy way to find such.

regards, tom lane

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