Address the N+1 Select Problem using JOIN FETCH

This is continuation to my previous post ‘Understanding the N+1 Select Problem’. In this post, I am going to show, how can we address N+1 select problem using FETCH JOIN annotation.

Example

public interface AuthorRepository extends JpaRepository<Author, Integer> {
	
	@Query("SELECT a FROM Author a JOIN FETCH a.books")
	List<Author> findAll();

}

By using JOIN FETCH in the query, the books associated with each author are loaded in the initial query itself, reducing the number of database queries and potential performance issues related to lazy loading and the N+1 select problem.

 

Find the below working application.

 

Step 1: Create new maven project ‘avoid-n-plus-one-problem-using-fetch-join’.

 

Step 2: Update pom.xml with maven dependencies.

 

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>com.sample.app</groupId>
	<artifactId>avoid-n-plus-one-problem-using-fetch-join</artifactId>
	<version>0.0.1-SNAPSHOT</version>

	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>3.2.2</version>
	</parent>

	<dependencies>

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-data-jpa</artifactId>
		</dependency>

		<dependency>
			<groupId>com.h2database</groupId>
			<artifactId>h2</artifactId>
		</dependency>

		<dependency>
			<groupId>com.zaxxer</groupId>
			<artifactId>HikariCP</artifactId>
		</dependency>

	</dependencies>

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>
</project>

Step 3: Define entity classes.

 

Author.java

package com.sample.app.entity;

import java.util.ArrayList;
import java.util.List;

import jakarta.persistence.CascadeType;
import jakarta.persistence.Entity;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.OneToMany;
import jakarta.persistence.Table;

@Entity
@Table(name="AUTHOR")
public class Author {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @OneToMany(mappedBy = "author", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Book> books = new ArrayList<>();

    public Author() {}

    public Author(String name) {
        this.name = name;
    }

    // Getters and Setters
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public List<Book> getBooks() {
        return books;
    }

    public void setBooks(List<Book> books) {
        this.books = books;
    }

    public void addBook(Book book) {
        books.add(book);
        book.setAuthor(this);
    }

    public void removeBook(Book book) {
        books.remove(book);
        book.setAuthor(null);
    }

	@Override
	public String toString() {
		return "Author [id=" + id + ", name=" + name + "]";
	}
    
    
}

Book.java

package com.sample.app.entity;

import jakarta.persistence.Entity;
import jakarta.persistence.FetchType;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;
import jakarta.persistence.Id;
import jakarta.persistence.JoinColumn;
import jakarta.persistence.ManyToOne;
import jakarta.persistence.Table;

@Entity
@Table(name = "BOOK")
public class Book {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String title;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "author_id")
    private Author author;

    public Book() {
    }

    public Book(String title) {
        this.title = title;
    }

    // Getters and Setters
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public Author getAuthor() {
        return author;
    }

    public void setAuthor(Author author) {
        this.author = author;
    }

    @Override
    public String toString() {
        return "Book [id=" + id + ", title=" + title + "]";
    }

}

Step 4: Define repository interfaces.

 

AuthorRepository.java

package com.sample.app.repository;

import java.util.List;

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Query;

import com.sample.app.entity.Author;

public interface AuthorRepository extends JpaRepository<Author, Integer> {
    
    @Query("SELECT a FROM Author a JOIN FETCH a.books")
    List<Author> findAll();

}

BookRepository.java

package com.sample.app.repository;

import org.springframework.data.jpa.repository.JpaRepository;

import com.sample.app.entity.Book;

public interface BookRepository extends JpaRepository<Book, Integer> {

}

Step 5: Define AuthorService class.

 

AuthorService.java

package com.sample.app.service;

import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

import com.sample.app.entity.Author;
import com.sample.app.entity.Book;
import com.sample.app.repository.AuthorRepository;

@Service
public class AuthorService {

    @Autowired
    private AuthorRepository authorRepository;

    @Transactional
    public void printAll() {
        List<Author> authors = authorRepository.findAll();

        for (Author author : authors) {
            System.out.println("author : " + author);
            for (Book book : author.getBooks()) {
                System.out.println("\t" + book);
            }
        }
    }
}

Step 6: Define DBConfig class.

 

DBConfig.java

package com.sample.app.config;

import com.zaxxer.hikari.HikariConfig;
import com.zaxxer.hikari.HikariDataSource;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import javax.sql.DataSource;
import java.io.IOException;

@Configuration
public class DBConfig {
    @Bean
    public DataSource dataSource() throws IOException {
        HikariConfig hikariConfig = new HikariConfig();
        hikariConfig.setDriverClassName("org.h2.Driver");
        hikariConfig.setJdbcUrl("jdbc:h2:mem:testdb;DB_CLOSE_DELAY=-1;DB_CLOSE_ON_EXIT=false");
        hikariConfig.setUsername("sa");
        hikariConfig.setPassword("");
        hikariConfig.setMaximumPoolSize(10);
        hikariConfig.setIdleTimeout(600000); // 10 minutes
        hikariConfig.setConnectionTimeout(30000); // 30 seconds
        hikariConfig.setMaxLifetime(1800000); // 30 minutes
        hikariConfig.setConnectionTestQuery("SELECT 1");
        hikariConfig.setValidationTimeout(5000); // 5 seconds
        hikariConfig.setAutoCommit(true);
        hikariConfig.setPoolName("springHikariCP");

        return new HikariDataSource(hikariConfig);
    }
}

Step 7: Define main application class.

 

App.java

package com.sample.app;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.transaction.annotation.Transactional;

import com.sample.app.entity.Author;
import com.sample.app.entity.Book;
import com.sample.app.repository.AuthorRepository;
import com.sample.app.service.AuthorService;

import jakarta.annotation.PostConstruct;

@SpringBootApplication
public class App {
    @Autowired
    private AuthorRepository authorRepository;
    
    @Autowired
    private AuthorService authorService;

    public static void main(String[] args) throws Exception {
        SpringApplication.run(App.class, args);
    }

    @PostConstruct
    @Transactional
    public void demo() {
        Author hari = new Author("Hari");
        hari.addBook(new Book("Hari Book 1"));
        hari.addBook(new Book("Hari Book 2"));

        Author ram = new Author("Ram");
        ram.addBook(new Book("Ram Book 1"));
        ram.addBook(new Book("Ram Book 2"));

        Author david = new Author("David");
        david.addBook(new Book("David Book 1"));
        david.addBook(new Book("David Book 2"));

        Author chandu = new Author("Chandu");
        chandu.addBook(new Book("David Book 1"));

        Author raghu = new Author("Raghu");
        raghu.addBook(new Book("Raghu Book 1"));

        authorRepository.save(hari);
        authorRepository.save(ram);
        authorRepository.save(david);
        authorRepository.save(chandu);
        authorRepository.save(raghu);

        authorService.printAll();
    }
}

Step 8: Create application.yml file under src/main/resources folder.

 

application.yml

spring:
  datasource:
    communicationtimeout: 60000
    jpa:
      hibernate:
        ddl-auto: create
  jpa:
    properties:
      hibernate:
        show_sql: true
        format_sql: false

Total project structure looks like below.

 

Build the Artifact

Navigate to the project where pom.xml file is located and execute below command.

mvn package

Upon successful execution of the command, you can see a jar file in the target folder. 

$ ls ./target/
avoid-n-plus-one-problem-using-fetch-join-0.0.1-SNAPSHOT.jar
avoid-n-plus-one-problem-using-fetch-join-0.0.1-SNAPSHOT.jar.original
classes
generated-sources
generated-test-sources
maven-archiver
maven-status
test-classes

Run the application

Execute below command to run the application.

java -jar ./target/avoid-n-plus-one-problem-using-fetch-join-0.0.1-SNAPSHOT.jar

 

You can see below messages in the console.

Hibernate: insert into author (name,id) values (?,default)
Hibernate: insert into book (author_id,title,id) values (?,?,default)
Hibernate: insert into book (author_id,title,id) values (?,?,default)
Hibernate: insert into author (name,id) values (?,default)
Hibernate: insert into book (author_id,title,id) values (?,?,default)
Hibernate: insert into book (author_id,title,id) values (?,?,default)
Hibernate: insert into author (name,id) values (?,default)
Hibernate: insert into book (author_id,title,id) values (?,?,default)
Hibernate: insert into book (author_id,title,id) values (?,?,default)
Hibernate: insert into author (name,id) values (?,default)
Hibernate: insert into book (author_id,title,id) values (?,?,default)
Hibernate: insert into author (name,id) values (?,default)
Hibernate: insert into book (author_id,title,id) values (?,?,default)
Hibernate: select a1_0.id,b1_0.author_id,b1_0.id,b1_0.title,a1_0.name from author a1_0 join book b1_0 on a1_0.id=b1_0.author_id
author : Author [id=1, name=Hari]
  Book [id=1, title=Hari Book 1]
  Book [id=2, title=Hari Book 2]
author : Author [id=2, name=Ram]
  Book [id=3, title=Ram Book 1]
  Book [id=4, title=Ram Book 2]
author : Author [id=3, name=David]
  Book [id=5, title=David Book 1]
  Book [id=6, title=David Book 2]
author : Author [id=4, name=Chandu]
  Book [id=7, title=David Book 1]
author : Author [id=5, name=Raghu]
  Book [id=8, title=Raghu Book 1]

 

You can download this application from this link.

  

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