Java.Hibernate.Middle.What is the N+1 select problem in Hibernate?

🔎 What is the N+1 select problem?

It happens when you fetch a collection of parent entities, and then Hibernate lazily loads each child entity in a separate query.

Specifically:
1️⃣ First, Hibernate executes 1 query to load your list of N parent entities.
2️⃣ Then, for each of those N parents, it issues 1 additional query to load the associated child entities.

So in total, you get 1 + N queries, which can be disastrous for performance if N is large.

📌 Example

Say you have Author entities, each with a collection of Book entities:

List<Author> authors = session.createQuery("FROM Author", Author.class).getResultList();

Hibernate first runs:

SELECT * FROM authors; -- 1 query

Then, for each author, it does:

SELECT * FROM books WHERE author_id = ?; -- N queries

So fetching 100 authors would result in 101 queries, which is inefficient and can slow your app significantly.

✅ How to solve it?

Use fetch join to load both parents and their children in a single query:

List<Author> authors = session.createQuery(
    "SELECT DISTINCT a FROM Author a JOIN FETCH a.books", Author.class
).getResultList();

This translates into:

SELECT a.*, b.* FROM authors a
JOIN books b ON a.id = b.author_id;

So you get 1 query instead of N+1, which greatly improves performance.

📝 Key points

✔ The problem stems from lazy loading of collections without batching or fetch joins.
✔ It’s called “N+1” because it literally does 1 query for the main list + N more queries for details.
✔ Solving it usually involves fetch joins, adjusting fetch strategies, or using batch fetching.