Suggested best setup for "Code" database table column

[musicollect]

Hello,

I'm a new user of nuBuilder using it for a project connecting to existing mySQL database tables.
Each table already has a primary key called "id", either int (11) or bigint (20). I've seen various references suggesting also using a database column called XXX_code (XXX = 3 character prefix taken from table name), where this "code" column is necessary within nuBuilder for cross-table lookups etc.

My question is, does this "code" column need to have a database index applied to it or is the primary key (id) used ?

Thanks for any advice

Greg