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JPA N+1 발생 원인 및 해결방법

Posted Feb 28, 2024 Updated Feb 28, 2024
By GangEun
22 min read

JPA를 사용한다면 한번쯤은 마주쳤던, 또는 들어 봤던 문제입니다.
바로 N+1 문제인데요. 원인과 해결방법을 알아보겠습니다.

N + 1 문제란?

JPA에서 연관 관계가 설정된 엔티티 사이에서 한 엔티티를 조회하였을 때 조회된 엔티티의 개수(N개)만큼
연관된 엔티티를 조회하기 위해추가적인 쿼리가 발생하는 (+1) 문제를 의미합니다.



지연로딩 & 즉시로딩

JPA에서 연관 관계를 설정할 때 먼저 알아야하는 필수적인 개념이 있습니다.
그것은 바로 Fetch Type 입니다.

FetchType 이란?

  • JPA가 하나의 Entity를 조회할 때, 연관관계에 있는 객체들을 어떻게 가져올 것인지에 대한 설정값
  • default값 -> xxToOne = EAGER, xxToMany = LAZY

즉시로딩 (Eager)

  • 데이터를 조회할 때, 연관된 모든 객체의 데이터까지 한번에 불러오는 것

지연로딩 (Lazy)

  • 데이터를 조회할 때는 프록시를 통해 가짜 객체를 넣어놓고 실제 조회 시점에 데이터를 불러오는 것



발생하는 상황

N+1 문제를 재현하기 위해 아래와 같이 연관관계를 걸어주었습니다.

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@Entity
public class Board {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "board_id")
    private Long id;

    private String title;

    private String content;

    private String writer;

    @OneToMany(mappedBy = "board", cascade = CascadeType.ALL)
    private List<Reply> replieList = new ArrayList<>();

}


@Entity
public class Reply {

  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  @Column(name = "reply_id")
  private Long id;

  private String content;

  @JoinColumn(name = "board_id")
  @ManyToOne(fetch = FetchType.LAZY)
  private Board board;

}

아래는 N + 1 문제 발생을 위한 테스트 코드입니다.

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    @DisplayName("N+1 테스트")
    @Test
    void test() {
        saveData();
        em.flush();
        em.clear();

        System.out.println("---------- BOARD 데이터 조회 ----------");
        List<Board> boardList = boardRepository.findAll();
        System.out.println("---------- BOARD 데이터 조회 완료 (1번의 쿼리) ---------- \n");


        System.out.println("---------- BOARD 제목, 내용, 작성자 조회 ----------");
        boardList.forEach(System.out::println);
        System.out.println("---------- BOARD 제목, 내용, 작성자 조회 완료 ----------\n");

        System.out.println("---------- BOARD 내부 REPLY 내용 조회 ----------");
        boardList.forEach(board -> {
            board.getReplieList().forEach(System.out::println);
        });
        System.out.println("---------- BOARD 내부 REPLY 내용 조회 완료 (N+1 문제 발생) ----------");


    }

    private void saveData() {
        IntStream.rangeClosed(1, 10).forEach(i -> {

            Board board = Board.builder()
                    .title(i + " board-title")
                    .content(i + " board-content")
                    .writer(i + " board-writer")
                    .build();

            IntStream.rangeClosed(1, 3).forEach(j -> {
                board.writerReply(j + " reply-content");
            });

            boardRepository.save(board);
        });
    }

실행 결과는 아래와 같습니다.

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---------- BOARD 데이터 조회 ----------
Hibernate: 
    select
        board0_.board_id as board_id1_0_,
        board0_.content as content2_0_,
        board0_.title as title3_0_,
        board0_.writer as writer4_0_ 
    from
        board board0_
---------- BOARD 데이터 조회 완료 (1번의 쿼리) ---------- 

---------- BOARD 제목, 내용, 작성자 조회 ----------
Board(id=172, title=1 board-title, content=1 board-content, writer=1 board-writer)
Board(id=173, title=2 board-title, content=2 board-content, writer=2 board-writer)
Board(id=174, title=3 board-title, content=3 board-content, writer=3 board-writer)
Board(id=175, title=4 board-title, content=4 board-content, writer=4 board-writer)
Board(id=176, title=5 board-title, content=5 board-content, writer=5 board-writer)
Board(id=177, title=6 board-title, content=6 board-content, writer=6 board-writer)
Board(id=178, title=7 board-title, content=7 board-content, writer=7 board-writer)
Board(id=179, title=8 board-title, content=8 board-content, writer=8 board-writer)
Board(id=180, title=9 board-title, content=9 board-content, writer=9 board-writer)
Board(id=181, title=10 board-title, content=10 board-content, writer=10 board-writer)
---------- BOARD 제목, 내용, 작성자 조회 완료 ----------

---------- BOARD 내부 REPLY 내용 조회 ----------
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Reply(id=496, content=1 reply-content, board=Board(id=172, title=1 board-title, content=1 board-content, writer=1 board-writer))
Reply(id=497, content=2 reply-content, board=Board(id=172, title=1 board-title, content=1 board-content, writer=1 board-writer))
Reply(id=498, content=3 reply-content, board=Board(id=172, title=1 board-title, content=1 board-content, writer=1 board-writer))
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Reply(id=499, content=1 reply-content, board=Board(id=173, title=2 board-title, content=2 board-content, writer=2 board-writer))
Reply(id=500, content=2 reply-content, board=Board(id=173, title=2 board-title, content=2 board-content, writer=2 board-writer))
Reply(id=501, content=3 reply-content, board=Board(id=173, title=2 board-title, content=2 board-content, writer=2 board-writer))
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Reply(id=502, content=1 reply-content, board=Board(id=174, title=3 board-title, content=3 board-content, writer=3 board-writer))
Reply(id=503, content=2 reply-content, board=Board(id=174, title=3 board-title, content=3 board-content, writer=3 board-writer))
Reply(id=504, content=3 reply-content, board=Board(id=174, title=3 board-title, content=3 board-content, writer=3 board-writer))
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Reply(id=505, content=1 reply-content, board=Board(id=175, title=4 board-title, content=4 board-content, writer=4 board-writer))
Reply(id=506, content=2 reply-content, board=Board(id=175, title=4 board-title, content=4 board-content, writer=4 board-writer))
Reply(id=507, content=3 reply-content, board=Board(id=175, title=4 board-title, content=4 board-content, writer=4 board-writer))
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Reply(id=508, content=1 reply-content, board=Board(id=176, title=5 board-title, content=5 board-content, writer=5 board-writer))
Reply(id=509, content=2 reply-content, board=Board(id=176, title=5 board-title, content=5 board-content, writer=5 board-writer))
Reply(id=510, content=3 reply-content, board=Board(id=176, title=5 board-title, content=5 board-content, writer=5 board-writer))
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Reply(id=511, content=1 reply-content, board=Board(id=177, title=6 board-title, content=6 board-content, writer=6 board-writer))
Reply(id=512, content=2 reply-content, board=Board(id=177, title=6 board-title, content=6 board-content, writer=6 board-writer))
Reply(id=513, content=3 reply-content, board=Board(id=177, title=6 board-title, content=6 board-content, writer=6 board-writer))
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Reply(id=514, content=1 reply-content, board=Board(id=178, title=7 board-title, content=7 board-content, writer=7 board-writer))
Reply(id=515, content=2 reply-content, board=Board(id=178, title=7 board-title, content=7 board-content, writer=7 board-writer))
Reply(id=516, content=3 reply-content, board=Board(id=178, title=7 board-title, content=7 board-content, writer=7 board-writer))
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Reply(id=517, content=1 reply-content, board=Board(id=179, title=8 board-title, content=8 board-content, writer=8 board-writer))
Reply(id=518, content=2 reply-content, board=Board(id=179, title=8 board-title, content=8 board-content, writer=8 board-writer))
Reply(id=519, content=3 reply-content, board=Board(id=179, title=8 board-title, content=8 board-content, writer=8 board-writer))
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Reply(id=520, content=1 reply-content, board=Board(id=180, title=9 board-title, content=9 board-content, writer=9 board-writer))
Reply(id=521, content=2 reply-content, board=Board(id=180, title=9 board-title, content=9 board-content, writer=9 board-writer))
Reply(id=522, content=3 reply-content, board=Board(id=180, title=9 board-title, content=9 board-content, writer=9 board-writer))
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Reply(id=523, content=1 reply-content, board=Board(id=181, title=10 board-title, content=10 board-content, writer=10 board-writer))
Reply(id=524, content=2 reply-content, board=Board(id=181, title=10 board-title, content=10 board-content, writer=10 board-writer))
Reply(id=525, content=3 reply-content, board=Board(id=181, title=10 board-title, content=10 board-content, writer=10 board-writer))
---------- BOARD 내부 REPLY 내용 조회 완료 (N+1 문제 발생) ----------

로그를 확인하면 Board 개수인 10개만큼의 추가적인 쿼리가 발생한걸 확인할 수 있습니다.
이 문제를 N + 1이라고 부릅니다.

그럼 이 문제는 지연로딩 에서만 발생할까요?
아닙니다. 즉시로딩 으로 변경하여 테스트 해보겠습니다.

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---------- BOARD 데이터 조회 ----------
Hibernate: 
    select
        board0_.board_id as board_id1_0_,
        board0_.content as content2_0_,
        board0_.title as title3_0_,
        board0_.writer as writer4_0_ 
    from
        board board0_
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.content as content2_1_1_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id=?
---------- BOARD 데이터 조회 완료 (1번의 쿼리) ---------- 

---------- BOARD 제목, 내용, 작성자 조회 ----------
Board(id=162, title=1 board-title, content=1 board-content, writer=1 board-writer)
Board(id=163, title=2 board-title, content=2 board-content, writer=2 board-writer)
Board(id=164, title=3 board-title, content=3 board-content, writer=3 board-writer)
Board(id=165, title=4 board-title, content=4 board-content, writer=4 board-writer)
Board(id=166, title=5 board-title, content=5 board-content, writer=5 board-writer)
Board(id=167, title=6 board-title, content=6 board-content, writer=6 board-writer)
Board(id=168, title=7 board-title, content=7 board-content, writer=7 board-writer)
Board(id=169, title=8 board-title, content=8 board-content, writer=8 board-writer)
Board(id=170, title=9 board-title, content=9 board-content, writer=9 board-writer)
Board(id=171, title=10 board-title, content=10 board-content, writer=10 board-writer)
---------- BOARD 제목, 내용, 작성자 조회 완료 ----------

---------- BOARD 내부 REPLY 내용 조회 ----------
Reply(id=466, content=1 reply-content, board=Board(id=162, title=1 board-title, content=1 board-content, writer=1 board-writer))
Reply(id=467, content=2 reply-content, board=Board(id=162, title=1 board-title, content=1 board-content, writer=1 board-writer))
Reply(id=468, content=3 reply-content, board=Board(id=162, title=1 board-title, content=1 board-content, writer=1 board-writer))
Reply(id=469, content=1 reply-content, board=Board(id=163, title=2 board-title, content=2 board-content, writer=2 board-writer))
Reply(id=470, content=2 reply-content, board=Board(id=163, title=2 board-title, content=2 board-content, writer=2 board-writer))
Reply(id=471, content=3 reply-content, board=Board(id=163, title=2 board-title, content=2 board-content, writer=2 board-writer))
Reply(id=472, content=1 reply-content, board=Board(id=164, title=3 board-title, content=3 board-content, writer=3 board-writer))
Reply(id=473, content=2 reply-content, board=Board(id=164, title=3 board-title, content=3 board-content, writer=3 board-writer))
Reply(id=474, content=3 reply-content, board=Board(id=164, title=3 board-title, content=3 board-content, writer=3 board-writer))
Reply(id=475, content=1 reply-content, board=Board(id=165, title=4 board-title, content=4 board-content, writer=4 board-writer))
Reply(id=476, content=2 reply-content, board=Board(id=165, title=4 board-title, content=4 board-content, writer=4 board-writer))
Reply(id=477, content=3 reply-content, board=Board(id=165, title=4 board-title, content=4 board-content, writer=4 board-writer))
Reply(id=478, content=1 reply-content, board=Board(id=166, title=5 board-title, content=5 board-content, writer=5 board-writer))
Reply(id=479, content=2 reply-content, board=Board(id=166, title=5 board-title, content=5 board-content, writer=5 board-writer))
Reply(id=480, content=3 reply-content, board=Board(id=166, title=5 board-title, content=5 board-content, writer=5 board-writer))
Reply(id=481, content=1 reply-content, board=Board(id=167, title=6 board-title, content=6 board-content, writer=6 board-writer))
Reply(id=482, content=2 reply-content, board=Board(id=167, title=6 board-title, content=6 board-content, writer=6 board-writer))
Reply(id=483, content=3 reply-content, board=Board(id=167, title=6 board-title, content=6 board-content, writer=6 board-writer))
Reply(id=484, content=1 reply-content, board=Board(id=168, title=7 board-title, content=7 board-content, writer=7 board-writer))
Reply(id=485, content=2 reply-content, board=Board(id=168, title=7 board-title, content=7 board-content, writer=7 board-writer))
Reply(id=486, content=3 reply-content, board=Board(id=168, title=7 board-title, content=7 board-content, writer=7 board-writer))
Reply(id=487, content=1 reply-content, board=Board(id=169, title=8 board-title, content=8 board-content, writer=8 board-writer))
Reply(id=488, content=2 reply-content, board=Board(id=169, title=8 board-title, content=8 board-content, writer=8 board-writer))
Reply(id=489, content=3 reply-content, board=Board(id=169, title=8 board-title, content=8 board-content, writer=8 board-writer))
Reply(id=490, content=1 reply-content, board=Board(id=170, title=9 board-title, content=9 board-content, writer=9 board-writer))
Reply(id=491, content=2 reply-content, board=Board(id=170, title=9 board-title, content=9 board-content, writer=9 board-writer))
Reply(id=492, content=3 reply-content, board=Board(id=170, title=9 board-title, content=9 board-content, writer=9 board-writer))
Reply(id=493, content=1 reply-content, board=Board(id=171, title=10 board-title, content=10 board-content, writer=10 board-writer))
Reply(id=494, content=2 reply-content, board=Board(id=171, title=10 board-title, content=10 board-content, writer=10 board-writer))
Reply(id=495, content=3 reply-content, board=Board(id=171, title=10 board-title, content=10 board-content, writer=10 board-writer))
---------- BOARD 내부 REPLY 내용 조회 완료 (N+1 문제 발생) ----------

N+1 쿼리가 나가는 시점만 달라졌을뿐 N+1문제가 발생하는걸 확인할 수 있습니다.



해결 방법

fetch join 사용

  • 조회의 주체가 되는 Entity 이외에 Fetch Join이 걸린 연관 Entity도 함께 SELECT 하여 모두 영속화
  • Fetch Join이 걸린 Entity 모두 영속화하기 때문에 FetchType이 Lazy인 Entity를 참조하더라도
    이미 영속성 컨텍스트에 들어있기 때문에 따로 쿼리가 실행되지 않은 채로 N+1문제가 해결됨
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    @Override
    @Query("SELECT distinct b FROM Board b JOIN FETCH b.replieList")
    List<Board> findAll();

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---------- BOARD 데이터 조회 ----------
Hibernate: 
    select
        distinct board0_.board_id as board_id1_0_0_,
        replielist1_.reply_id as reply_id1_1_1_,
        board0_.content as content2_0_0_,
        board0_.title as title3_0_0_,
        board0_.writer as writer4_0_0_,
        replielist1_.board_id as board_id3_1_1_,
        replielist1_.content as content2_1_1_,
        replielist1_.board_id as board_id3_1_0__,
        replielist1_.reply_id as reply_id1_1_0__ 
    from
        board board0_ 
    inner join
        reply replielist1_ 
            on board0_.board_id=replielist1_.board_id
---------- BOARD 데이터 조회 완료 (1번의 쿼리) ---------- 

---------- BOARD 제목, 내용, 작성자 조회 ----------
Board(id=212, title=1 board-title, content=1 board-content, writer=1 board-writer)
Board(id=213, title=2 board-title, content=2 board-content, writer=2 board-writer)
Board(id=214, title=3 board-title, content=3 board-content, writer=3 board-writer)
Board(id=215, title=4 board-title, content=4 board-content, writer=4 board-writer)
Board(id=216, title=5 board-title, content=5 board-content, writer=5 board-writer)
Board(id=217, title=6 board-title, content=6 board-content, writer=6 board-writer)
Board(id=218, title=7 board-title, content=7 board-content, writer=7 board-writer)
Board(id=219, title=8 board-title, content=8 board-content, writer=8 board-writer)
Board(id=220, title=9 board-title, content=9 board-content, writer=9 board-writer)
Board(id=221, title=10 board-title, content=10 board-content, writer=10 board-writer)
---------- BOARD 제목, 내용, 작성자 조회 완료 ----------

---------- BOARD 내부 REPLY 내용 조회 ----------
Reply(id=616, content=1 reply-content, board=Board(id=212, title=1 board-title, content=1 board-content, writer=1 board-writer))
Reply(id=617, content=2 reply-content, board=Board(id=212, title=1 board-title, content=1 board-content, writer=1 board-writer))
Reply(id=618, content=3 reply-content, board=Board(id=212, title=1 board-title, content=1 board-content, writer=1 board-writer))
Reply(id=619, content=1 reply-content, board=Board(id=213, title=2 board-title, content=2 board-content, writer=2 board-writer))
Reply(id=620, content=2 reply-content, board=Board(id=213, title=2 board-title, content=2 board-content, writer=2 board-writer))
Reply(id=621, content=3 reply-content, board=Board(id=213, title=2 board-title, content=2 board-content, writer=2 board-writer))
Reply(id=622, content=1 reply-content, board=Board(id=214, title=3 board-title, content=3 board-content, writer=3 board-writer))
Reply(id=623, content=2 reply-content, board=Board(id=214, title=3 board-title, content=3 board-content, writer=3 board-writer))
Reply(id=624, content=3 reply-content, board=Board(id=214, title=3 board-title, content=3 board-content, writer=3 board-writer))
Reply(id=625, content=1 reply-content, board=Board(id=215, title=4 board-title, content=4 board-content, writer=4 board-writer))
Reply(id=626, content=2 reply-content, board=Board(id=215, title=4 board-title, content=4 board-content, writer=4 board-writer))
Reply(id=627, content=3 reply-content, board=Board(id=215, title=4 board-title, content=4 board-content, writer=4 board-writer))
Reply(id=628, content=1 reply-content, board=Board(id=216, title=5 board-title, content=5 board-content, writer=5 board-writer))
Reply(id=629, content=2 reply-content, board=Board(id=216, title=5 board-title, content=5 board-content, writer=5 board-writer))
Reply(id=630, content=3 reply-content, board=Board(id=216, title=5 board-title, content=5 board-content, writer=5 board-writer))
Reply(id=631, content=1 reply-content, board=Board(id=217, title=6 board-title, content=6 board-content, writer=6 board-writer))
Reply(id=632, content=2 reply-content, board=Board(id=217, title=6 board-title, content=6 board-content, writer=6 board-writer))
Reply(id=633, content=3 reply-content, board=Board(id=217, title=6 board-title, content=6 board-content, writer=6 board-writer))
Reply(id=634, content=1 reply-content, board=Board(id=218, title=7 board-title, content=7 board-content, writer=7 board-writer))
Reply(id=635, content=2 reply-content, board=Board(id=218, title=7 board-title, content=7 board-content, writer=7 board-writer))
Reply(id=636, content=3 reply-content, board=Board(id=218, title=7 board-title, content=7 board-content, writer=7 board-writer))
Reply(id=637, content=1 reply-content, board=Board(id=219, title=8 board-title, content=8 board-content, writer=8 board-writer))
Reply(id=638, content=2 reply-content, board=Board(id=219, title=8 board-title, content=8 board-content, writer=8 board-writer))
Reply(id=639, content=3 reply-content, board=Board(id=219, title=8 board-title, content=8 board-content, writer=8 board-writer))
Reply(id=640, content=1 reply-content, board=Board(id=220, title=9 board-title, content=9 board-content, writer=9 board-writer))
Reply(id=641, content=2 reply-content, board=Board(id=220, title=9 board-title, content=9 board-content, writer=9 board-writer))
Reply(id=642, content=3 reply-content, board=Board(id=220, title=9 board-title, content=9 board-content, writer=9 board-writer))
Reply(id=643, content=1 reply-content, board=Board(id=221, title=10 board-title, content=10 board-content, writer=10 board-writer))
Reply(id=644, content=2 reply-content, board=Board(id=221, title=10 board-title, content=10 board-content, writer=10 board-writer))
Reply(id=645, content=3 reply-content, board=Board(id=221, title=10 board-title, content=10 board-content, writer=10 board-writer))
---------- BOARD 내부 REPLY 내용 조회 완료 (N+1 문제 발생) ----------

데이터를 한번에 가져와 영속화 시켜 N+1 문제가 발생되지 않습니다.

@EntityGraph 사용

  • 단순하게 fetch 조인을 어노테이션으로 사용할 수 있도록 만들어준 기능이다.
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    @Override
    @EntityGraph(attributePaths = "replieList")
    List<Board> findAll();

결과 및 발생 쿼리는 fetch join과 동일합니다.

BatchSize

  • IN 쿼리를 통해 지정한 사이즈만큼 한번에 가져오는 것
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@BatchSize(size = 100)
@OneToMany(mappedBy = "board", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
private List<Reply> replieList = new ArrayList<>();

위와 같이 Entity에 직접 설정 또는 yml 파일에 속성 지정 가능
spring.jpa.properties.hibernate.default_batch_fetch_size

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---------- BOARD 데이터 조회 ----------
Hibernate: 
    select
        board0_.board_id as board_id1_0_,
        board0_.content as content2_0_,
        board0_.title as title3_0_,
        board0_.writer as writer4_0_ 
    from
        board board0_
---------- BOARD 데이터 조회 완료 (1번의 쿼리) ---------- 

---------- BOARD 제목, 내용, 작성자 조회 ----------
Board(id=252, title=1 board-title, content=1 board-content, writer=1 board-writer)
Board(id=253, title=2 board-title, content=2 board-content, writer=2 board-writer)
Board(id=254, title=3 board-title, content=3 board-content, writer=3 board-writer)
Board(id=255, title=4 board-title, content=4 board-content, writer=4 board-writer)
Board(id=256, title=5 board-title, content=5 board-content, writer=5 board-writer)
Board(id=257, title=6 board-title, content=6 board-content, writer=6 board-writer)
Board(id=258, title=7 board-title, content=7 board-content, writer=7 board-writer)
Board(id=259, title=8 board-title, content=8 board-content, writer=8 board-writer)
Board(id=260, title=9 board-title, content=9 board-content, writer=9 board-writer)
Board(id=261, title=10 board-title, content=10 board-content, writer=10 board-writer)
---------- BOARD 제목, 내용, 작성자 조회 완료 ----------

---------- BOARD 내부 REPLY 내용 조회 ----------
Hibernate: 
    select
        replielist0_.board_id as board_id3_1_1_,
        replielist0_.reply_id as reply_id1_1_1_,
        replielist0_.reply_id as reply_id1_1_0_,
        replielist0_.board_id as board_id3_1_0_,
        replielist0_.content as content2_1_0_ 
    from
        reply replielist0_ 
    where
        replielist0_.board_id in (
            ?, ?, ?, ?, ?, ?, ?, ?, ?, ?
        )
Reply(id=736, content=1 reply-content, board=Board(id=252, title=1 board-title, content=1 board-content, writer=1 board-writer))
Reply(id=737, content=2 reply-content, board=Board(id=252, title=1 board-title, content=1 board-content, writer=1 board-writer))
Reply(id=738, content=3 reply-content, board=Board(id=252, title=1 board-title, content=1 board-content, writer=1 board-writer))
Reply(id=739, content=1 reply-content, board=Board(id=253, title=2 board-title, content=2 board-content, writer=2 board-writer))
Reply(id=740, content=2 reply-content, board=Board(id=253, title=2 board-title, content=2 board-content, writer=2 board-writer))
Reply(id=741, content=3 reply-content, board=Board(id=253, title=2 board-title, content=2 board-content, writer=2 board-writer))
Reply(id=742, content=1 reply-content, board=Board(id=254, title=3 board-title, content=3 board-content, writer=3 board-writer))
Reply(id=743, content=2 reply-content, board=Board(id=254, title=3 board-title, content=3 board-content, writer=3 board-writer))
Reply(id=744, content=3 reply-content, board=Board(id=254, title=3 board-title, content=3 board-content, writer=3 board-writer))
Reply(id=745, content=1 reply-content, board=Board(id=255, title=4 board-title, content=4 board-content, writer=4 board-writer))
Reply(id=746, content=2 reply-content, board=Board(id=255, title=4 board-title, content=4 board-content, writer=4 board-writer))
Reply(id=747, content=3 reply-content, board=Board(id=255, title=4 board-title, content=4 board-content, writer=4 board-writer))
Reply(id=748, content=1 reply-content, board=Board(id=256, title=5 board-title, content=5 board-content, writer=5 board-writer))
Reply(id=749, content=2 reply-content, board=Board(id=256, title=5 board-title, content=5 board-content, writer=5 board-writer))
Reply(id=750, content=3 reply-content, board=Board(id=256, title=5 board-title, content=5 board-content, writer=5 board-writer))
Reply(id=751, content=1 reply-content, board=Board(id=257, title=6 board-title, content=6 board-content, writer=6 board-writer))
Reply(id=752, content=2 reply-content, board=Board(id=257, title=6 board-title, content=6 board-content, writer=6 board-writer))
Reply(id=753, content=3 reply-content, board=Board(id=257, title=6 board-title, content=6 board-content, writer=6 board-writer))
Reply(id=754, content=1 reply-content, board=Board(id=258, title=7 board-title, content=7 board-content, writer=7 board-writer))
Reply(id=755, content=2 reply-content, board=Board(id=258, title=7 board-title, content=7 board-content, writer=7 board-writer))
Reply(id=756, content=3 reply-content, board=Board(id=258, title=7 board-title, content=7 board-content, writer=7 board-writer))
Reply(id=757, content=1 reply-content, board=Board(id=259, title=8 board-title, content=8 board-content, writer=8 board-writer))
Reply(id=758, content=2 reply-content, board=Board(id=259, title=8 board-title, content=8 board-content, writer=8 board-writer))
Reply(id=759, content=3 reply-content, board=Board(id=259, title=8 board-title, content=8 board-content, writer=8 board-writer))
Reply(id=760, content=1 reply-content, board=Board(id=260, title=9 board-title, content=9 board-content, writer=9 board-writer))
Reply(id=761, content=2 reply-content, board=Board(id=260, title=9 board-title, content=9 board-content, writer=9 board-writer))
Reply(id=762, content=3 reply-content, board=Board(id=260, title=9 board-title, content=9 board-content, writer=9 board-writer))
Reply(id=763, content=1 reply-content, board=Board(id=261, title=10 board-title, content=10 board-content, writer=10 board-writer))
Reply(id=764, content=2 reply-content, board=Board(id=261, title=10 board-title, content=10 board-content, writer=10 board-writer))
Reply(id=765, content=3 reply-content, board=Board(id=261, title=10 board-title, content=10 board-content, writer=10 board-writer))
---------- BOARD 내부 REPLY 내용 조회 완료 (N+1 문제 발생) ----------

위와같이 in 절을 톻애 데이터를 가져와 N +1 문제가 발생하지 않는것을 확인할 수 있습니다.



마치며

사실 위 fetch join, @EntityGraph는 @xxxToOne 관계에서만 적용하는게 현실적입니다.
다시 한번 fetch join 쿼리를 확인해 보시면 제가 distinct 를 해준것을 볼 수 있습니다.
@xxxToMany 관계에서 fetch join 및 EntityGraph를 적용하면 페이징 처리 불가능 및 데이터 부정합 현상 이 발생됩니다.

참고로 BatchSize는 어느 관계에서든 적용가능합니다.

  • @xxxToOne : fetch join or @EntityGraph or batch size 적용
  • @xxxToMany : batch size 적용
spring, JPA
java 자바 JPA N+1 연관관계 fetch
This post is written by PRO.
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