Query

Hi All, I have two questions

1- I have a query but this is very slow, I need to acelerate the
consult, this result sill be show in the web.

My query is:

select user, descripcion from client where login not in (select
distinct(usr_cliente) from conexion where fecha between '$date_begin' and
'$date_end') order by user"

How can I to acelerate this consult?

2- I made this function:

CREATE FUNCTION list(date,date) RETURNS user
AS 'select distinct(usr_cliente) from conexion where fecha between
$date_begin and $date_end' LANGUAGE 'sql';

When I execute a query to this function like:
select select listado('08-10-2001', '08-13-2001') as user;

the result is just 1 row, however If I to execute the query this return
300 rows, why the function only return one row?

Thanks in advanced,

Regards,