"Specified key was too long. Max key length is 500" with a ALTER statement

[Stefan Hartmann]

Hello.

I have a problem with a MySQL query which don't work at my test server. It worked at several servers, but not at an old XAMPP. I search the reason and a workaround for it.

This is my table

CREATE TABLE IF NOT EXISTS `wa_configuration` (
`id` bigint(21) NOT NULL auto_increment,
`name` varchar(255) NOT NULL default '',
`value` varchar(255) NOT NULL default '',
`module` varchar(255) NOT NULL default '',
PRIMARY KEY (`id`)
) TYPE=MyISAM AUTO_INCREMENT=1 ;

When I execute

ALTER TABLE `wa_configuration` ADD UNIQUE `name_and_module` (`name`, `module`)

I get the error

Specified key was too long. Max key length is 500

I use MySQL Client 5.0.67, Server 3.23.58-max-debug at a XAMPP for Win ME. (I use that system for downward compatibility tests, NOT as working system).

I read about the problem when using UTF8 in varchar(255) tables. But in that MySQL3, I think, that's not the reason. Why is there an error at ALTER TABLE while adding an unique symbol?

What is the reason and is there a workaround for this?

(Because my application is simple, I want to keep MySQL3, Apache1 and PHP4 support to it, so I am testing it at old systems to make it friendly to them)

Regards
S. Hartmann