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From: "Josh Berkus" <josh@agliodbs.com>
Subject: Re: index on to_char(created, 'YYYY') doesn't work
To: Steve Crawford <scrawford@pinpointresearch.com>,
	Andreas Joseph Krogh <andreak@officenet.no>,
	Achilleus Mantzios <achill@matrix.gatewaynet.com>
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Steve,

> BTW, I tried to create an index on the to_char function and had no
> luck - 
> seems like it should work but it doesn't on 7.2.3 or 7.3.1.

That's because functional indexes can't take any arguments other than
column names.   Therefore you'd need to:

CREATE FUNCTION to_year (timestamp) RETURNS varchar AS
'SELECT to_char($1, ''YYYY'');'
LANGUAGE 'sql' WITH (ISCACHABLE, ISSTRICT)
(above is 7.2.3 syntax)

Then use the to_year function in place of to_char for creating your
index.

-Josh Berkus