How to setup postfix to access to the database with procedures

Hi to everybody.

I've realized a mailserver with postfix, using this how-to: http://www.howtoforge.com/virtual-users-domains-postfix-courier-mysql-squirrelmail-debian-lenny

It's working fine, but for the final configuration I need to access to the mysql database via procedures...
So I created the procedures like that:

Code:

DELIMITER $$

DROP PROCEDURE IF EXISTS `get_virtualrelaydomains` $$
CREATE DEFINER=`xxxxxx`@`%` PROCEDURE `get_virtualrelaydomains`(IN domain1 VARCHAR(45))
BEGIN
SELECT domain FROM mail_transport WHERE domain = domain1;
END $$

DELIMITER ;

and the .cf files like this:

Code:

user = xxxxx
password = xxxxxx
dbname = xxxxxx
query = CALL get_virtualrelaydomains('%s')
hosts = 127.0.0.1

but it's not working... it's giving me this kind of problems:

Code:

 postfix/proxymap[9339]: warning: mysql query failed: PROCEDURE xxxx.get_virtualrelaydomains can't return a result set in the given context

etc...

Any idea how to setup postfix in this way?

Thanks
Michele