When a big database is shared by many applications, it is sometimes hard to understand who is using what, and what would happen if the database schema changes. In that case, it makes sense to build the whole system using layered architecture. The physical data structure is located at the bottom-most layer. Applications do not access it directly.
Moving upwards from the bottom, the second last layer contains structures that abstract logical entities from their physical implementation. These structures play the role of data abstraction interfaces. There are several ways to implement them. They can be created in the database as functions. In that case, applications will work with the data by invoking them. Another approach is by using updatable views. In that case, applications can access the logical entities with conventional SQL statements. Additionally, this interface can be implemented outside the database as a lightweight service processing the requests of high level...