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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alex Hunsaker <badalex@gmail.com>
Cc: Zdenek Kotala <Zdenek.Kotala@sun.com>
Cc: Xiao Meng <mx.cogito@gmail.com>
Cc: pgsql-patches@postgresql.org, "Jonah H. Harris" <jonah.harris@gmail.com>
Subject: Re: hash index improving v3
Date: Thu, 04 Sep 2008 22:17:07 -0400
Message-ID: <11962.1220581027@sss.pgh.pa.us> (raw)
In-Reply-To: <34d269d40809041854g355a8f68ga3a037a69e3d9fcb@mail.gmail.com>
References: <ded849dd0808171846g2c6c65adub942bd2510a6c94f@mail.gmail.com>
	<48BFDA8D.3080506@sun.com>
	<29987.1220560114@sss.pgh.pa.us>
	<7627.1220569888@sss.pgh.pa.us>
	<34d269d40809041742r6a0703dbgfbdd012f7f19de2e@mail.gmail.com>
	<10995.1220577198@sss.pgh.pa.us>
	<11525.1220579153@sss.pgh.pa.us>
	<34d269d40809041854g355a8f68ga3a037a69e3d9fcb@mail.gmail.com>

"Alex Hunsaker" <badalex@gmail.com> writes:
> On Thu, Sep 4, 2008 at 7:45 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> 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.

> Hrm, well I have not really looked at the hash algorithm but I assume
> we could just reduce the number of buckets?

No, we need fully equal hash keys, else the code won't visit the heap.

I guess one thing we could do for testing purposes is lobotomize one of
the datatype-specific hash functions.  For instance, make int8_hash
return the input mod 2^32, ignoring the upper bytes.  Then it'd be easy
to compute different int8s that hash to the same thing.

			regards, tom lane



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