인프런 커뮤니티 질문&답변

첫번째 JPQL

Query("select m from Member m join m.team ")
List<Member> findInnerJoin();

    @Test
    public void test() {
        Team teamA = new Team("teamA");
        Team teamB = new Team("teamB");
        teamRepository.save(teamA);
        teamRepository.save(teamB);
        Member member1 = new Member(10, "member1", teamA);
        Member member2 = new Member(10, "member2", teamB);
        memberRepository.save(member1);
        memberRepository.save(member2);

        em.flush();
        em.clear();

        List<Member> members = memberRepository.findInnerJoin();
        for (Member member : members) {
            System.out.println("member = " + member);
            System.out.println("member.getTeam().getClass() = " + member.getTeam().getClass());
            System.out.println("member.getTeam().getName() = " + member.getTeam().getName());
        }
    }

실행결과

Hibernate: 
    select
        m1_0.member_id,
        m1_0.age,
        m1_0.team_id,
        m1_0.username 
    from
        member m1_0 
    join
        team t1_0 
            on t1_0.team_id=m1_0.team_id
member = Member(id=1, username=member1, age=10)
member.getTeam().getClass() = class study.data_jpa.entity.Team$HibernateProxy$w0d2fXxq
Hibernate: 
    select
        t1_0.team_id,
        t1_0.name 
    from
        team t1_0 
    where
        t1_0.team_id=?
member.getTeam().getName() = teamA
member = Member(id=2, username=member2, age=10)
member.getTeam().getClass() = class study.data_jpa.entity.Team$HibernateProxy$w0d2fXxq
Hibernate: 
    select
        t1_0.team_id,
        t1_0.name 
    from
        team t1_0 
    where
        t1_0.team_id=?
member.getTeam().getName() = teamB

첫번째 JPQL에서는 전형적인 N + 1 문제가 발생함을 알 수 있습니다. 그래서 첫번째 JPQL에서의 select문에 t만 추가를 해봤습니다.

두번째 JPQL

@Query("select m, t from Member m join m.team t")
List<Member> findInnerJoin();

실행결과

Hibernate: 
    select
        m1_0.member_id,
        m1_0.age,
        m1_0.team_id,
        m1_0.username,
        t1_0.team_id,
        t1_0.name 
    from
        member m1_0 
    join
        team t1_0 
            on t1_0.team_id=m1_0.team_id
member = Member(id=1, username=member1, age=10)
member.getTeam().getClass() = class study.data_jpa.entity.Team
member.getTeam().getName() = teamA
member = Member(id=2, username=member2, age=10)
member.getTeam().getClass() = class study.data_jpa.entity.Team
member.getTeam().getName() = teamB

위 실행 결과를 보고 페치 조인과 무슨 차이가 있을까 해서 페치 조인을 테스트 해봤습니다.

세번째 JPQL

@Query("select m from Member m join fetch m.team t")
List<Member> findMemberFetchJoinTeam();

@Test
public void ManyToOneFetchJoin() {
    Team teamA = new Team("teamA");
    Team teamB = new Team("teamB");
    teamRepository.save(teamA);
    teamRepository.save(teamB);
    Member member1 = new Member(10, "member1", teamA);
    Member member2 = new Member(10, "member2", teamB);
    memberRepository.save(member1);
    memberRepository.save(member2);

    em.flush();
    em.clear();

    List<Member> members = memberRepository.findMemberFetchJoinTeam();
    for (Member member : members) {
        System.out.println("member = " + member);
        System.out.println("member.getTeam().getClass() = " + member.getTeam().getClass());
        System.out.println("member.getTeam().getName() = " + member.getTeam().getName());
    }
}

실행결과

Hibernate: 
    select
        m1_0.member_id,
        m1_0.age,
        t1_0.team_id,
        t1_0.name,
        m1_0.username 
    from
        member m1_0 
    join
        team t1_0 
            on t1_0.team_id=m1_0.team_id
member = Member(id=1, username=member1, age=10)
member.getTeam().getClass() = class study.data_jpa.entity.Team
member.getTeam().getName() = teamA
member = Member(id=2, username=member2, age=10)
member.getTeam().getClass() = class study.data_jpa.entity.Team
member.getTeam().getName() = teamB

두번째 JPQL과 세번째 JPQL의 차이를 보면 두번째 JPQL의 select문에서 m1_0.team_id 도 조회하고 세번째 JPQL의 select문에서는 m1_0.team_id 을 조회하지 않습니다.

두번째 JPQL과 세번째 JPQL이 정확히 어떤 차이가 있어 select문에서 조회하는 컬럼이 달라지는지 궁금합니다.