Subject: Re: LIKE query on indexes
From: Scott Marlowe <smarlowe@g2switchworks.com>
To: Alvaro Herrera <alvherre@commandprompt.com>
Cc: Ibrahim Tekin <itekin@gmail.com>, pgsql-performance@postgresql.org
In-Reply-To: <20060221163416.GC6541@surnet.cl>
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	<1140538679.5777.3.camel@state.g2switchworks.com>
	<20060221163416.GC6541@surnet.cl>
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Date: Tue, 21 Feb 2006 10:42:24 -0600

On Tue, 2006-02-21 at 10:34, Alvaro Herrera wrote:
> Scott Marlowe wrote:
> > On Tue, 2006-02-21 at 09:57, Ibrahim Tekin wrote:
> > > hi,
> > > i have btree index on a text type field. i want see rows which starts
> > > with certain characters on that field. so i write a query like this:
> > > 
> > > SELECT * FROM mytable WHERE myfield LIKE 'john%'
> > > 
> > > since this condition is from start of the field, query planner should
> > > use index to find such elements but explain command shows me it will
> > > do a sequential scan. 
> > > 
> > > is this lack of a feature or i am wrong somewhere?
> > 
> > This is an artifact of how PostgreSQL handles locales other than ASCII.
> > 
> > If you want such a query to use an index, you need to back up your
> > database, and re-initdb with --locale=C as an argument.
> 
> ... or you can choose to create an index with the text_pattern_ops
> operator class, which would be used in a LIKE constraint regardless of
> locale.
> 
> http://www.postgresql.org/docs/8.1/static/indexes-opclass.html

Good point.  I tend to view the world from the perspective of the 7.4
and before user...