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To: Andreas Joseph Krogh <andreak@officenet.no>
cc: pgsql-sql@postgresql.org
Subject: Re: PG won't use index on ORDER BY <expression> 
In-reply-to: <200708100015.34117.andreak@officenet.no> 
References: <200708092018.44111.andreak@officenet.no>
	<87fy2s1nrd.fsf@oxford.xeocode.com>
	<200708092238.46284.andreak@officenet.no>
	<200708100015.34117.andreak@officenet.no>
Comments: In-reply-to Andreas Joseph Krogh <andreak@officenet.no>
	message dated "Fri, 10 Aug 2007 00:15:33 +0200"
Date: Thu, 09 Aug 2007 19:35:08 -0400
Message-ID: <3878.1186702508@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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Andreas Joseph Krogh <andreak@officenet.no> writes:
>> Another question then: Why doesn't "varchar_pattern_ops" handle ordering?

It does handle ordering, just not the ordering you're asking for here.
If you substituted USING ~<~ for ASC you'd find that the pattern_ops
index could be used for that.

>> This means I need 2 indexes on the columns I want to match with LIKE and
>> ORDER BY. Just doesn't seem right to need 2 "similar" indexes...

If you want to use the same index for both, you have to run the database
in C locale.  Non-C locales generally define a sort ordering that is not
compatible with LIKE searches.  (The point of the pattern_ops opclass is
really to force C-locale ordering of the index when the ordinary text
comparison operators yield a different ordering.)

> -- This doesn't
> EXPLAIN ANALYZE select firstname, lastname from person order by
> concat_lower(firstname, lastname) ASC, created DESC limit 10;

This ORDER BY is asking for an ordering that is almost completely
unrelated to the index's ordering.

			regards, tom lane