Problema com consultas N + 1 em JPA e Hibernate

Antes do curso "Highload Architect", convidamos você a participar do tutorial aberto sobre "Scale-out Storage Patterns" .

Nesse ínterim, compartilhamos a tradução tradicional de um artigo útil.

Introdução

Neste artigo, explicarei qual é o problema de consulta N + 1 com JPA e Hibernate e a melhor forma de corrigi-lo. 

O problema N + 1 não é específico para JPA e Hibernate e você pode encontrá-lo ao usar outras tecnologias de acesso a dados.

Qual é o problema N + 1

O problema N + 1 ocorre quando a estrutura de acesso a dados executa N consultas SQL adicionais para obter os mesmos dados que podem ser obtidos em uma única consulta SQL.

Quanto maior o valor de N, mais consultas serão executadas e maior será o impacto no desempenho. E embora o log de consultas lentas possa ajudá-lo a encontrar consultas lentas, ele não detectará o problema N + 1, uma vez que cada consulta adicional separada é executada com rapidez suficiente. 

, , .

: post () post_comments ( ), "--":

post :

INSERT INTO post (title, id)
VALUES ('High-Performance Java Persistence - Part 1', 1)
  
INSERT INTO post (title, id)
VALUES ('High-Performance Java Persistence - Part 2', 2)
  
INSERT INTO post (title, id)
VALUES ('High-Performance Java Persistence - Part 3', 3)
  
INSERT INTO post (title, id)
VALUES ('High-Performance Java Persistence - Part 4', 4)

post_comment :

INSERT INTO post_comment (post_id, review, id)
VALUES (1, 'Excellent book to understand Java Persistence', 1)
  
INSERT INTO post_comment (post_id, review, id)
VALUES (2, 'Must-read for Java developers', 2)
  
INSERT INTO post_comment (post_id, review, id)
VALUES (3, 'Five Stars', 3)
  
INSERT INTO post_comment (post_id, review, id)
VALUES (4, 'A great reference book', 4)

N+1 SQL

, N + 1 , SQL.

post_comments SQL-:

List<Tuple> comments = entityManager.createNativeQuery("""
    SELECT
        pc.id AS id,
        pc.review AS review,
        pc.post_id AS postId
    FROM post_comment pc
    """, Tuple.class)
.getResultList();

(title) (post) (post_comment):

for (Tuple comment : comments) {
    String review = (String) comment.get("review");
    Long postId = ((Number) comment.get("postId")).longValue();
 
    String postTitle = (String) entityManager.createNativeQuery("""
        SELECT
            p.title
        FROM post p
        WHERE p.id = :postId
        """)
    .setParameter("postId", postId)
    .getSingleResult();
 
    LOGGER.info(
        "The Post '{}' got this review '{}'",
        postTitle,
        review
    );
}

N + 1, SQL- (1 + 4):

SELECT
    pc.id AS id,
    pc.review AS review,
    pc.post_id AS postId
FROM post_comment pc
 
SELECT p.title FROM post p WHERE p.id = 1
-- The Post 'High-Performance Java Persistence - Part 1' got this review
-- 'Excellent book to understand Java Persistence'
    
SELECT p.title FROM post p WHERE p.id = 2
-- The Post 'High-Performance Java Persistence - Part 2' got this review
-- 'Must-read for Java developers'
     
SELECT p.title FROM post p WHERE p.id = 3
-- The Post 'High-Performance Java Persistence - Part 3' got this review
-- 'Five Stars'
     
SELECT p.title FROM post p WHERE p.id = 4
-- The Post 'High-Performance Java Persistence - Part 4' got this review
-- 'A great reference book'

N + 1 . , , SQL-, , :

List<Tuple> comments = entityManager.createNativeQuery("""
    SELECT
        pc.id AS id,
        pc.review AS review,
        p.title AS postTitle
    FROM post_comment pc
    JOIN post p ON pc.post_id = p.id
    """, Tuple.class)
.getResultList();
 
for (Tuple comment : comments) {
    String review = (String) comment.get("review");
    String postTitle = (String) comment.get("postTitle");
 
    LOGGER.info(
        "The Post '{}' got this review '{}'",
        postTitle,
        review
    );
}

SQL- , .

N + 1 JPA Hibernate

JPA Hibernate N + 1, , .

, post post_comments:

JPA- :

@Entity(name = "Post")
@Table(name = "post")
public class Post {
 
    @Id
    private Long id;
 
    private String title;
 
    //Getters and setters omitted for brevity
}
 
@Entity(name = "PostComment")
@Table(name = "post_comment")
public class PostComment {
 
    @Id
    private Long id;
 
    @ManyToOne
    private Post post;
 
    private String review;
 
    //Getters and setters omitted for brevity
}

FetchType.EAGER

FetchType.EAGER JPA- — , , . , FetchType.EAGER N + 1. 

, @ManyToOne @OneToOne FetchType.EAGER, , :

@ManyToOne
private Post post;

FetchType.EAGER ,  JOIN FETCH PostComment JPQL- Criteria API:

List<PostComment> comments = entityManager
.createQuery("""
    select pc
    from PostComment pc
    """, PostComment.class)
.getResultList();

N + 1:

SELECT
    pc.id AS id1_1_,
    pc.post_id AS post_id3_1_,
    pc.review AS review2_1_
FROM
    post_comment pc
 
SELECT p.id AS id1_0_0_, p.title AS title2_0_0_ FROM post p WHERE p.id = 1
SELECT p.id AS id1_0_0_, p.title AS title2_0_0_ FROM post p WHERE p.id = 2
SELECT p.id AS id1_0_0_, p.title AS title2_0_0_ FROM post p WHERE p.id = 3
SELECT p.id AS id1_0_0_, p.title AS title2_0_0_ FROM post p WHERE p.id = 4

SELECT, , PostComment post.

, find EntityManager, JPQL- Criteria API (fetch plan), Hibernate , JOIN FETCH. , .

post, : FetchType.EAGER . FetchType.LAZY.

post, JOIN FETCH, N + 1:

List<PostComment> comments = entityManager.createQuery("""
    select pc
    from PostComment pc
    join fetch pc.post p
    """, PostComment.class)
.getResultList();
 
for(PostComment comment : comments) {
    LOGGER.info(
        "The Post '{}' got this review '{}'",
        comment.getPost().getTitle(),
        comment.getReview()
    );
}

Hibernate SQL-:

SELECT
    pc.id as id1_1_0_,
    pc.post_id as post_id3_1_0_,
    pc.review as review2_1_0_,
    p.id as id1_0_1_,
    p.title as title2_0_1_
FROM
    post_comment pc
INNER JOIN
    post p ON pc.post_id = p.id
     
-- The Post 'High-Performance Java Persistence - Part 1' got this review
-- 'Excellent book to understand Java Persistence'
 
-- The Post 'High-Performance Java Persistence - Part 2' got this review
-- 'Must-read for Java developers'
 
-- The Post 'High-Performance Java Persistence - Part 3' got this review
-- 'Five Stars'
 
-- The Post 'High-Performance Java Persistence - Part 4' got this review
-- 'A great reference book'

, FetchType.EAGER, .

FetchType.LAZY

FetchType.LAZY , N + 1.

post :

@ManyToOne(fetch = FetchType.LAZY)
private Post post;

, PostComment:

List<PostComment> comments = entityManager
.createQuery("""
    select pc
    from PostComment pc
    """, PostComment.class)
.getResultList();

Hibernate SQL-:

SELECT
    pc.id AS id1_1_,
    pc.post_id AS post_id3_1_,
    pc.review AS review2_1_
FROM
    post_comment pc

lazy-load post:

for(PostComment comment : comments) {
    LOGGER.info(
        "The Post '{}' got this review '{}'",
        comment.getPost().getTitle(),
        comment.getReview()
    );
}

N + 1 :

SELECT p.id AS id1_0_0_, p.title AS title2_0_0_ FROM post p WHERE p.id = 1
-- The Post 'High-Performance Java Persistence - Part 1' got this review
-- 'Excellent book to understand Java Persistence'
 
SELECT p.id AS id1_0_0_, p.title AS title2_0_0_ FROM post p WHERE p.id = 2
-- The Post 'High-Performance Java Persistence - Part 2' got this review
-- 'Must-read for Java developers'
 
SELECT p.id AS id1_0_0_, p.title AS title2_0_0_ FROM post p WHERE p.id = 3
-- The Post 'High-Performance Java Persistence - Part 3' got this review
-- 'Five Stars'
 
SELECT p.id AS id1_0_0_, p.title AS title2_0_0_ FROM post p WHERE p.id = 4
-- The Post 'High-Performance Java Persistence - Part 4' got this review
-- 'A great reference book'

post , SQL- .

, JOIN FETCH JPQL:

List<PostComment> comments = entityManager.createQuery("""
    select pc
    from PostComment pc
    join fetch pc.post p
    """, PostComment.class)
.getResultList();
 
for(PostComment comment : comments) {
    LOGGER.info(
        "The Post '{}' got this review '{}'",
        comment.getPost().getTitle(),
        comment.getReview()
    );
}

FetchType.EAGER, JPQL- SQL-.

FetchType.LAZY @OneToOne, N + 1.

, N+1 c @OneToOne-, .

N + 1 .

, JPQL-, :

List<PostComment> comments = entityManager.createQuery("""
    select pc
    from PostComment pc
    order by pc.post.id desc
    """, PostComment.class)
.setMaxResults(10)
.setHint(QueryHints.HINT_CACHEABLE, true)
.getResultList();

PostComment , N PostComment:

-- Checking cached query results in region: org.hibernate.cache.internal.StandardQueryCache
-- Checking query spaces are up-to-date: [post_comment]
-- [post_comment] last update timestamp: 6244574473195524, result set timestamp: 6244574473207808
-- Returning cached query results
  
SELECT pc.id AS id1_1_0_,
       pc.post_id AS post_id3_1_0_,
       pc.review AS review2_1_0_
FROM post_comment pc
WHERE pc.id = 3
  
SELECT pc.id AS id1_1_0_,
       pc.post_id AS post_id3_1_0_,
       pc.review AS review2_1_0_
FROM post_comment pc
WHERE pc.id = 2
  
SELECT pc.id AS id1_1_0_,
       pc.post_id AS post_id3_1_0_,
       pc.review AS review2_1_0_
FROM post_comment pc
WHERE pc.id = 1

PostComment. , PostComment , N SQL-.

"Highload Architect".