N+1 QUERY
PROBLEM

The N+1 query problem occurs because of how ORM frameworks such as Hibernate ORM (used by Spring Boot through Spring Data JPA) load entity relationships.

Case Discussion

Assume the typical mapping:

@Entity
class Order {
    @OneToMany(mappedBy = "order", fetch = FetchType.LAZY)
    private List<OrderItem> items;
}

Step-by-step execution of your code:

List<Order> orders = orderRepository.findAll();

The ORM executes one SQL query:

SELECT * FROM orders;

Suppose this returns N orders.

Example result:

| order_id |
| -------- |
| 1        |
| 2        |
| 3        |

At this point items are not loaded yet because the relationship uses LAZY loading.

Then your code accesses:

order.getItems()

For each order, Hibernate must load the associated items. Because they were not fetched earlier, it executes one additional query per order:

SELECT * FROM order_items WHERE order_id = 1;
SELECT * FROM order_items WHERE order_id = 2;
SELECT * FROM order_items WHERE order_id = 3;

Total queries executed:

1 query -> load orders
N queries -> load items for each order
-------------------------------
N + 1 queries

Example with 100 orders:

1 query -> SELECT orders
100 queries -> SELECT items by order_id
--------------------------------------
101 queries total

This is inefficient because:

• check_circle Many round trips to the database
• check_circle Increased latency
• check_circle Poor scalability under load

Why the ORM behaves this way:

• check_circle @OneToMany defaults to LAZY fetching
• check_circle The ORM loads relations only when accessed
• check_circle Accessing the collection triggers a separate query

Typical ways to solve it: