function runs slow

I have a query:

SELECT date_observed FROM tbl_a

WHERE
x = 384394918 AND
y = 5 AND
date_observed <= '14-Jul-10 00:00'
ORDER BY
date_observed DESC
LIMIT
1;

This query returns in 16 ms. Great! So I functionized the working method as:

CREATE OR REPLACE FUNCTION get_last_on_or_before(c integer, t integer, g timestamp with time zone)
RETURNS timestamp with time zone AS
$BODY$

SELECT date_observed FROM tbl_a
WHERE
x = $1 AND
y = $2 AND
date_observed <= $3
ORDER BY
date_observed DESC
LIMIT
1;

$BODY$
LANGUAGE 'sql' VOLATILE
COST 100;

Then I queried this new function:

SELECT get_last_on_or_before(384394918, 5, '14-Jul-10 00:00');

The query returns in 2891 ms! This is too slow for the application.

Why is it slow when the same un-functionized query was fast? The tbl_a has est. 30,000,000 records. It has an index for (x,y) pairs. It has an index for date_observed. So good results on the straight query. Where did I go wrong with the function?

Charlie

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