The N+1 query problem

JPA loads collections lazily by default, only when you first touch them. That is where the N+1 problem comes from. You fetch 50 orders and walk each order's items, in a loop or while turning them into JSON.

50 extra selects run. @ManyToOne is eager by default, which causes its own surprises. So mark relations LAZY and fetch what you need on purpose.

Fix N+1 with JOIN FETCH in a JPQL query, or an @EntityGraph on the repository method. Or use a DTO projection, a query that selects only the fields the response needs. Turn on SQL logging in development with spring.jpa.show-sql or Hibernate statistics.