Use auto_increment in non primary columns

[0ctanium]

What would you like to be added or enhanced
It would be useful to be able to auto increment a field but not setting it as a primary key.

Why is this needed
In the example of the european invoicing model, the invoice number increment need to follow the rest.

But in a data model where you use a unique id to refrenence the table in the overall model, you cannot use the uid column has primary key along with the invoice number with an auto increment