Fetch type controls when an association is loaded: EAGER fetches it immediately with the parent, LAZY defers it until you actually touch the field. LAZY is the right default for nearly everything, and "why EAGER is almost always wrong" is one of the most common JPA interview questions.
How LAZY works
With LAZY, Hibernate doesn't load the association up front. For a @ManyToOne/@OneToOne it returns a proxy (a subclass with the PK set); for a collection it returns a wrapper. The real SELECT fires only when you call a method on it.
Order o = repo.findById(1L).orElseThrow(); // one SELECT, customer not loaded
o.getCustomer().getName(); // triggers the SELECT now (if in session)Why EAGER is almost always wrong
Three concrete failures:
1. You pay for data you don't use. EAGER fetches the association on every load of the parent, even in queries that never touch it. A findAll() of orders drags every customer along, every time.
2. It silently causes N+1. When you load a list of parents with an EAGER @ManyToOne, Hibernate may issue one extra SELECT per parent to fetch each association — the N+1 problem, except you didn't ask for it and can't easily turn it off.
3. EAGER can't be turned off per query, but LAZY can be turned on. Fetch type set to EAGER on the mapping is global; you can't make it lazy for one query. LAZY is the opposite — it's the cheap default, and when a specific query needs the association, you opt in with JOIN FETCH or @EntityGraph. That asymmetry is the whole argument: default to LAZY, fetch eagerly per query.
@Query("SELECT o FROM Order o JOIN FETCH o.customer WHERE o.id = :id")
Optional<Order> findWithCustomer(@Param("id") Long id);The catch: LazyInitializationException
LAZY's price is that touching a lazy field after the persistence context closes throws LazyInitializationException — the proxy has no session to load from. The fix is not "make it EAGER" (or spring.jpa.open-in-view=true, which is on by default and hides the bug). The fix is to fetch what you need inside the transaction — via JOIN FETCH, @EntityGraph, or by mapping straight to a DTO.