CHAR() or VARCHAR() as primary key in an ISAM MySQL table?

Question

I need a simple table with a user name and password field in MySQL. Since user names must be unique, it makes sense to me to make them the primary key.

Is it better

Answer 1

I would work hard to NOT use CHAR() or VARCHAR() as a PK but use an int with an auto_increment instead. This allows you to use that user_id in child tables if needed and queries on the PK should be faster. If you have to use either a CHAR() or VARCHAR(), I'd go with the CHAR() since it's a fixed width.

I'm not 100% sure how MySQL deals with VARCHAR()'s but most database engines have to do some magic under the hood to help the engine know where the VARCHAR() fields ends and where the next field begins, a CHAR() makes it straight forward and keeps the engine from having to think to much.