Postgresql function to generate unique alpha-numeric codes

alphanumeric_code.pgsql

-- Actually, if UNIQUE is defined on the col, there is no need to store previous codes
-- It will fail to insert due to uniqueness contraint when all possible codes are exhausted
-- Use as a column default...
--     CREATE TABLE foo (id CHAR(6) DEFAULT AlphaNumericSerial() UNIQUE);

CREATE OR REPLACE FUNCTION AlphaNumericSerial() 
RETURNS char(6) AS $$
DECLARE _serial char(6); _i int; _chars char(36) = 'abcdefghijklmnopqrstuvwxyz0123456789';
BEGIN
    _serial = '';
    FOR _i in 1 .. 6 LOOP
        _serial = _serial || substr(_chars, int4(random() * length(_chars)), 1);
    END LOOP;
    RETURN lower(_serial);
END;
$$ LANGUAGE plpgsql VOLATILE;

good, thanks guys