benedicte_asselin wrote:

> another question: is there a penalty if expressing both constraints?

As it turns out, there are two ways to make a field unique - one is to
declare a unique index (old fashioned) and the other is to declare a
unique constraint (modern SQL). The constraint causes the creation of a
unique index, for performance. In order to create a foreign key
constraint referencing a field, you must have a unique constraint - not
just an index - on that field.

And no, there's no performance penalty for declaring both constraints.


Regards,


Ann