If I have this table, function and index in Postgres 7.3.6 ...

CREATE TABLE news_stories (
    id serial primary key NOT NULL,
    pub_date timestamp with time zone NOT NULL,
CREATE OR REPLACE FUNCTION get_year_trunc(timestamp with time zone) returns 
timestamp with time zone AS 'SELECT date_trunc(\'year\',$1);' LANGUAGE 'SQL' 
CREATE INDEX news_stories_pub_date_year_trunc ON 
news_stories( get_year_trunc(pub_date) );
...why does this query not use the index?
db=# EXPLAIN SELECT DISTINCT get_year_trunc(pub_date) FROM news_stories;
                                   QUERY PLAN
 Unique  (cost=59597.31..61311.13 rows=3768 width=8)
   ->  Sort  (cost=59597.31..60454.22 rows=342764 width=8)
         Sort Key: date_trunc('year'::text, pub_date)
         ->  Seq Scan on news_stories  (cost=0.00..23390.55 rows=342764 
(4 rows)

The query is noticably slow (2 seconds) on a database with 150,000+ records. 
How can I speed it up?


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