Improper Usage Of @Fetch(FetchMode.JOIN) May Causes N+1 Issues

Advice: Always evaluate JOIN FETCH and entities graphs before deciding to use FetchMode.JOIN. The FetchMode.JOIN fetch mode always triggers an EAGER load so the children are loaded when the parents are. Beside this drawback, FetchMode.JOIN may return duplicate results. You’ll have to remove the duplicates yourself (e.g. storing the result in a Set). But, if you decide to go with FetchMode.JOIN at least pay attention to avoid N+1 issues discussed below.

Note: Let's assume three entities, Author, Book and Publisher. Between Author and Book there is a bidirectional-lazy @OneToMany association. Between Author and Publisher there is a unidirectional-lazy @ManyToOne. Between Book and Publisher there is no association.

Now, we want to fetch a book by id (BookRepository#findById()), including its author, and the author's publisher. In such cases, using Hibernate fetch mode, @Fetch(FetchMode.JOIN) works as expected. Using JOIN FETCH or entity graph is also working as expected.

Next, we want to fetch all books (BookRepository#findAll()), including their authors, and the authors publishers. In such cases, using Hibernate fetch mode, @Fetch(FetchMode.JOIN) will cause N+1 issues. It will not trigger the expected JOIN. In this case, using JOIN FETCH or entity graph should be used.

Key points:

• using Hibernate fetch mode, @Fetch(FetchMode.JOIN) doesn't work for query-methods
• Hibernate fetch mode, @Fetch(FetchMode.JOIN) works in cases that fetches the entity by id (primary key) like using EntityManager#find(), Spring Data, findById(), findOne().

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