Mysql foreign key by non unique key — how is that possible?

Question

I was migrating mysql database to postgres and stumbled across the following block in DDL (Note: This is what I got from mysqldump):

CREATE TABLE `catalog_pr         


        

Answer 1

The most likely answer is that id really is unique in the catalog_propery_value table, but that the author declared the PK to be the superkey (id, sort) for reasons unknown, possibly having to do with indexing, rather than enforcing uniqueness.