What data loading mechanism is used when employing JOIN FETCH in ORM?

JOIN FETCH in ORM (for example, in JPA/Hibernate) is used for eager loading of related entities in a single query.

Mechanism:

• When executing a query with JOIN FETCH, ORM performs an SQL query with a JOIN that immediately loads the main entity and its related objects.
• This prevents the "N+1 query problem," where a separate query is executed for each related entity.
• The loaded related objects are immediately available in memory without additional database access.

Example:

String jpql = "SELECT o FROM Order o JOIN FETCH o.items WHERE o.id = :id";
Order order = entityManager.createQuery(jpql, Order.class)
    .setParameter("id", orderId)
    .getSingleResult();

Here, items will be loaded together with Order in a single query.

Thus, JOIN FETCH implements eager loading of related data by joining tables in one query.