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From: Mark Lewis <mark.lewis@mir3.com>
To: 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 09:30:37 -0800
Message-ID: <1140197437.9076.268.camel@archimedes> (raw)
In-Reply-To: <Pine.LNX.4.62.0602162131211.20790@qnivq.ynat.uz>
References: <43F38867.6010701@gpdnet.co.uk>
	<19510.1140036968@sss.pgh.pa.us>
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	<43F39E53.1020009@gpdnet.co.uk>
	<20781.1140046109@sss.pgh.pa.us>
	<7.0.1.0.2.20060215194635.03b55da0@earthlink.net>
	<21615.1140052893@sss.pgh.pa.us>
	<7.0.1.0.2.20060215223710.03a7c618@earthlink.net>
	<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>

On Thu, 2006-02-16 at 21:33 -0800, David Lang wrote:
> > In SQL_ASCII, just take the first 4 characters (or 8, if using a 64-bit
> > sortKey as elsewhere suggested).  The sorting key doesn't need to be a
> > one-to-one mapping.
> 
> that would violate your second contraint ( f(a)==f(b) iff (a==b) )
> 
> if you could drop that constraint (the cost of which would be extra 'real' 
> compares within a bucket) then a helper function per datatype could work 
> as you are talking.

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)

Which doesn't imply 'if and only if'.  It's a similar constraint to
hashcodes; the same value will always have the same hash, but you're not
guaranteed that the hashcodes for two distinct values will be unique.

-- Mark



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  Cc: mark.lewis@mir3.com, dlang@invendra.net, gsstark@mit.edu, schabi@logix-tt.com, pgsql-hackers@postgresql.org
  Subject: Re: qsort again (was Re: [PERFORM] Strange Create Index
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