Indexes in PL/SQL

I'm using PgSQL 7.3.2 and a PL/SQL routine that is posing some performance
problems for me. I have a table of users information as such (trimmed down):

user_main (
    uid int4,
    username character varying(32),
    password character varying(16)
);

Primary Index is uid
Unique Index on username

There are approx 1.5million records in the table.

Now, on the psql command line when i do

    SELECT uid FROM user_main WHERE username='bob';

it works and is very fast - the data is returned almost as fast as I can hit
enter.

Now I have a PL/SQL function (pgsql mail check):

    CREATE FUNCTION valid_user(text) RETURNS text AS '
    DECLARE
        _user ALIAS FOR $1;
        _uid user_main.uid%TYPE;
    BEGIN
        SELECT uid INTO _uid FROM user_main WHERE username=_user;

        IF FOUND THEN
            RETURN ''OK'';
        END IF;

        RETURN ''554 Sender Address Rejected'';
    END;' LANGUAGE 'plpgsql';

It works, however, the select is taking a very long time, 5-10 seconds.

Explain doesn't help much since it's a PL/SQL routine and I could not find
any mention of this in the lists (maybe my search criteria sucked).

Any ideas?

--
Tony Holmes

Founder and Senior Systems Architect
Crosswinds Internet Communications Inc.