N+1 query problem in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the association annotation to fetch associated customers eagerly when fetching orders.

Spring Boot

@ManyToOne(fetch = [1])
private Customer customer;

Drag options to blanks, or click blank then click option'

AFetchType.EAGER

BFetchType.LAZY

CFetchType.DEFAULT

DFetchType.NONE

Attempts:

3 left

💡 Hint

Common Mistakes

Using LAZY fetch causes multiple queries for each related entity.

Using DEFAULT or NONE does not explicitly solve the N+1 problem.

Using FetchType.EAGER ensures that related entities are loaded immediately, preventing the N+1 query problem.

2fill in blank

medium

Complete the JPQL query to fetch orders with customers using a join fetch to avoid N+1 queries.

Spring Boot

SELECT o FROM Order o [1] JOIN FETCH o.customer

Drag options to blanks, or click blank then click option'

ALEFT

BFULL

CINNER

DRIGHT

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT JOIN FETCH might include orders without customers.

RIGHT and FULL joins are rarely used in JPQL and may cause errors.

Using INNER JOIN FETCH fetches orders and their customers in one query, avoiding N+1 queries.

3fill in blank

hard

Fix the error in the repository method to prevent N+1 queries by adding the correct annotation.

Spring Boot

@Query("SELECT o FROM Order o [1] JOIN FETCH o.customer")
List<Order> findAllOrdersWithCustomers();

Drag options to blanks, or click blank then click option'

AINNER

BLEFT

CRIGHT

DOUTER

Attempts:

3 left

💡 Hint

Common Mistakes

Using RIGHT or OUTER join fetch causes JPQL syntax errors.

Omitting JOIN FETCH causes lazy loading and N+1 queries.

The INNER JOIN FETCH annotation in the query ensures orders and customers are fetched together, avoiding N+1 queries.

4fill in blank

hard

Fill both blanks to define a Spring Data JPA method that fetches orders with customers eagerly.

Spring Boot

List<Order> findAllBy[1]Fetch[2]();

Drag options to blanks, or click blank then click option'

ACustomer

BOrder

CEagerly

DLazy

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Order' in the first blank is incorrect because the method targets customers.

Using 'Lazy' in the second blank does not solve N+1 queries.

The method name findAllByCustomerFetchEagerly suggests fetching orders with customers eagerly, helping avoid N+1 queries.

5fill in blank

hard

Fill all three blanks to complete the code that uses EntityGraph to solve the N+1 query problem.

Spring Boot

@EntityGraph(attributePaths = {"[1]"})
List<Order> findAllWith[2]();

// Usage:
List<Order> orders = orderRepository.[3]();

Drag options to blanks, or click blank then click option'

Acustomer

BCustomers

CfindAllWithCustomers

DfindAll

Attempts:

3 left

💡 Hint

Common Mistakes

Using plural 'customer' in attributePaths causes errors.

Calling 'findAll()' does not use the EntityGraph and causes N+1 queries.

Using @EntityGraph(attributePaths = {"customer"}) with method findAllWithCustomers() fetches orders with customers eagerly, avoiding N+1 queries.

Practice

1. What is the N+1 query problem in Spring Boot applications?

easy

A. Not using any database queries at all

B. Making only one query to fetch all data including related entities

C. Using incorrect SQL syntax in queries

D. Making one query to fetch a list, then one query per item to fetch related data

Solution

1. Step 1: Understand the query pattern

   The N+1 problem occurs when the app first fetches a list (1 query), then fetches related data for each item separately (N queries).

2. Step 2: Identify the problem impact

   This causes many queries, slowing down the app and wasting resources.

3. Final Answer:

   Making one query to fetch a list, then one query per item to fetch related data -> Option D

4. Quick Check:

   N+1 query problem = multiple queries instead of one [OK]

Hint: N+1 means 1 query + N queries for related data [OK]

Common Mistakes:

• Thinking N+1 means only one query is made
• Confusing it with syntax errors
• Assuming it is about missing queries

2. Which of the following is the correct way to use JOIN FETCH in a Spring Data JPA query to avoid the N+1 problem?

easy

A. @Query("SELECT o FROM Order o JOIN FETCH o.items")

B. @Query("SELECT o FROM Order o JOIN o.items")

C. @Query("SELECT o FROM Order o LEFT JOIN o.items")

D. @Query("SELECT o FROM Order o WHERE o.items IS NOT NULL")

Solution

1. Step 1: Understand JOIN FETCH usage

   JOIN FETCH tells JPA to fetch related entities eagerly in one query, avoiding multiple queries.

2. Step 2: Identify correct syntax

   @Query("SELECT o FROM Order o JOIN FETCH o.items") uses JOIN FETCH correctly to fetch orders with their items in one query.

3. Final Answer:

   @Query("SELECT o FROM Order o JOIN FETCH o.items") -> Option A

4. Quick Check:

   JOIN FETCH = eager fetch to avoid N+1 [OK]

Hint: Use JOIN FETCH to load related data in one query [OK]

Common Mistakes:

• Using JOIN without FETCH causes lazy loading
• Using WHERE instead of JOIN FETCH
• Missing FETCH keyword

3. Given this Spring Data JPA repository method:

@Query("SELECT c FROM Customer c")
List<Customer> findAllCustomers();

And assuming Customer has a lazy-loaded orders collection, what happens when you call findAllCustomers() and then access orders for each customer?

medium

A. One query to get customers, then one query per customer to get orders (N+1 problem)

B. One query to get customers and all orders in one go

C. No queries are made until orders are accessed

D. An error occurs because orders are not fetched

Solution

1. Step 1: Analyze the query and lazy loading

   The query fetches customers only; orders are lazy-loaded, so not fetched initially.

2. Step 2: Accessing orders triggers queries

   Accessing orders for each customer triggers one query per customer, causing N+1 queries total.

3. Final Answer:

   One query to get customers, then one query per customer to get orders (N+1 problem) -> Option A

4. Quick Check:

   Lazy loading causes N+1 queries [OK]

Hint: Lazy loading causes one query per item when accessed [OK]

Common Mistakes:

• Assuming all data loads in one query
• Thinking no queries run until orders accessed
• Confusing lazy and eager loading

4. You have this code snippet causing N+1 queries:

List<Author> authors = authorRepository.findAll();
for (Author a : authors) {
    System.out.println(a.getBooks().size());
}

How can you fix it to avoid the N+1 problem?

medium

A. Add @Transactional annotation to the method

B. Call getBooks() inside a separate thread

C. Change repository method to use @Query("SELECT a FROM Author a JOIN FETCH a.books")

D. Remove the loop and print authors only

Solution

1. Step 1: Identify cause of N+1

   Calling getBooks() inside loop triggers one query per author due to lazy loading.

2. Step 2: Use JOIN FETCH to load books eagerly

   Changing repository query to use JOIN FETCH loads authors and books in one query, avoiding N+1.

3. Final Answer:

   Change repository method to use @Query("SELECT a FROM Author a JOIN FETCH a.books") -> Option C

4. Quick Check:

   JOIN FETCH fixes N+1 by eager loading [OK]

Hint: Use JOIN FETCH in query to load related data eagerly [OK]

Common Mistakes:

• Adding @Transactional does not fix N+1
• Using threads does not solve query count
• Removing loop hides problem but does not fix it

5. You have entities Post and Comment with a one-to-many lazy relationship. You want to fetch all posts with their comments efficiently. Which approach best avoids the N+1 problem and handles posts with no comments?

hard

A. Use native SQL without JOIN FETCH and map manually

B. @Query("SELECT p FROM Post p LEFT JOIN FETCH p.comments") to fetch posts and comments in one query

C. Fetch posts first, then fetch comments in a separate query for each post

D. Fetch posts only and ignore comments to reduce queries

Solution

1. Step 1: Understand lazy loading and N+1

   Lazy loading comments causes one query per post when accessed, causing N+1 problem.

2. Step 2: Use LEFT JOIN FETCH to include posts without comments

   LEFT JOIN FETCH fetches posts and their comments in one query, including posts with no comments.

3. Final Answer:

   @Query("SELECT p FROM Post p LEFT JOIN FETCH p.comments") to fetch posts and comments in one query -> Option B

4. Quick Check:

   LEFT JOIN FETCH avoids N+1 and includes empty collections [OK]

Hint: Use LEFT JOIN FETCH to include all posts and comments [OK]

Common Mistakes:

• Using INNER JOIN FETCH excludes posts without comments
• Fetching comments separately causes N+1
• Ignoring comments loses needed data