How To Optimize The Merge Operation Using Update

Description: Behind the built-in Spring Data save() there is a call of EntityManager#persist() or EntityManager#merge(). It is important to know this aspect in several cases. Among this cases, we have the entity update case (simple update or update batching).

Consider Author and Book involved in a bidirectional-lazy @OneToMany association. And, we load an Author, detach it, update it in the detached state, and save it to the database via save() method. Calling save() will come with the following two issues resulting from calling merge() behind the scene:

• there will be two SQL statements, one SELECT (merge) and one UPDATE
• the SELECT will contain a LEFT OUTER JOIN to fetch the associated Book as well (we don't need the books!)

How about triggering only the UPDATE instead of this? The solution relies on calling Session#update(). Calling Session.update() requires to unwrap the Session via entityManager.unwrap(Session.class).

Key points:

• calling Session.update() will trigger only the UPDATE (there is no SELECT)
• Session.update() works with versioned optimistic locking mechanism as well (so, lost updates are prevented)