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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Mark Lewis <mark.lewis@mir3.com>
Cc: David Lang <dlang@invendra.net>
Cc: Greg Stark <gsstark@mit.edu>
Cc: Markus Schaber <schabi@logix-tt.com>
Cc: pgsql-performance@postgresql.org
Cc: pgsql-hackers@postgresql.org
Subject: Re: qsort again (was Re: [PERFORM] Strange Create Index
Date: Fri, 17 Feb 2006 14:43:06 -0500
Message-ID: <11801.1140205386@sss.pgh.pa.us> (raw)
In-Reply-To: <1140197437.9076.268.camel@archimedes>
References: <43F38867.6010701@gpdnet.co.uk>
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	<7.0.1.0.2.20060215194635.03b55da0@earthlink.net>
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	<20060216113522.GA3461@uio.no>
	<7.0.1.0.2.20060216071053.0385a1a8@earthlink.net>
	<43F481CD.2010509@logix-tt.com>
	<43F4A7D8.4050907@modgraph-usa.com>
	<29277.1140110108@sss.pgh.pa.us>
	<1140128256.9076.254.camel@archimedes>
	<43F4FDCC.2000204@logix-tt.com>
	<871wy23okp.fsf@stark.xeocode.com>
	<1140132189.9076.261.camel@archimedes>
	<Pine.LNX.4.62.0602162131211.20790@qnivq.ynat.uz>
	<1140197437.9076.268.camel@archimedes>

Mark Lewis <mark.lewis@mir3.com> writes:
> I think we're actually on the same page here; you're right that the
> constraint above ( f(a)==f(b) iff a==b ) can't be extended to data types
> with more than 32 bits of value space.  But the constraint I listed was
> actually:

> if a==b then f(a)==f(b)

I believe Martijn had it right: the important constraint is

	f(a) > f(b) implies a > b

which implies by commutativity

	f(a) < f(b) implies a < b

and these two together imply

	a == b implies f(a) == f(b)

Now you can't do any sorting if you only have the equality rule, you
need the inequality rule.

			regards, tom lane

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