What is the N+1 query problem, and how do you find and fix it?

N+1 happens when code loads N entities with one query and then touches a lazy association on each, so Hibernate runs one extra query per entity:

List<Author> authors = authorRepository.findAll();          // 1 query
for (Author a : authors) {
    System.out.println(a.getBooks().size());                 // + 1 query per author
}

With 10 test rows nobody notices; with 5,000 authors it’s 5,001 round trips.

Fixes, depending on the use case:

• Fetch join or @EntityGraph: load the association in the same query (select a from Author a left join fetch a.books). Don’t combine a collection fetch join with paging: Hibernate then pages in memory.
• Batch fetching (hibernate.default_batch_fetch_size=50): lazy loads are grouped into where author_id in (…) queries, turning N+1 into N/50+1 without changing code.
• DTO projections: for read-only endpoints, select exactly the columns you need, often with aggregation (count(b)), and no entities at all.

Detection: set spring.jpa.open-in-view=false so lazy loading in the web layer fails loudly, enable Hibernate statistics or a datasource proxy in integration tests, and assert a maximum number of statements per request.