Let's imagine a data model with customers and invoices. There is a 1 to n relationship between a customer and its invoices. We uses an ORM (like Hibernate).
One can explicitely implement the 1-n relationship (using JPA for example) or not. If not, then one must do a bit more work to fetch invoices.
However, it is much easier to maintain, improve and develop the data model of applications where relationships between objects are not explicitely implemented in the database.
My question is, has anyone noticed a significant performance impact when not implementing the relationships in the database?