It’d be great if the primary field for a table could be defined as consisting of multiple fields. For example, I have a “Movies Watched” base containing a table listing movies, and another table listing viewings. The viewings table has a link to the movies table and a date. Each viewing record can be uniquely identified by the combination of the movie and date. At present, since there is only one primary field, and that field cannot be a lookup, I have to set the primary field as a formula concatenating the movie lookup and date to create a fake composite field.
A (simpler) alternative would be allowing the primary field to be hidden - as if the primary field is a formula based on other fields (such as in this instance) the data making up this primary field is already displayed elsewhere, and any fields used to create that primary field cannot be hidden if you want to be able to (easily) edit them.