Why varchar for primary keys?

[einar-hjortdal]

Medusa uses a prefix and an ULID as entity id as primary key for every entity.

1. Why was character varying chosen as type to store these primary keys?
2. For what reason is an ULID preferred over a simple integer?
   • Do entity ids have to match ids across other databases?
   • Must these entity ids be generated by medusa instead of letting the database generate them?