[Spring] QueryDSL 쿼리문 비교

1. left join & fetch join

연관된 데이터를 즉시 로딩하기 위해 사용, 이를 사용하면 하이버네이트는 연관된 데이터를 함께 가져오도록 쿼리문을 생성, 나중에 접근할 때 발생할 수 있는 추가쿼리를 방지

• post 엔티티와 관련된 user 엔티티를 패치조인으로 연결, post 데이터를 로드할 때 관련된 user 데이터도 함께 즉시 로딩

< 중요 - 레프트 조인만 한거랑 패치조인한거랑 쿼리 같아보이는 문제 왜? >

• 두 쿼리 모두 post 데이터를 기준으로 조회하므로 결과집합 구조는 동일하다. 즉 얻어지는 post 리스트는 동일하다.
• 결과가 시각적으로는 같지만 데이터에 접근하는 방식에서 차이가 있다. 패치조인을 사용한 경우에는 post 를 가져올 때 관련 user 데이터도 이미 로드되어 있으므로 데이터 접근 시 지연이 없다, 하지만 레프트조인만 사용한 경우에는 user 데이터에 처음 접근할 때 추가 로드가 필요할 수 있으 성능차이가 발생할 수 있다.

select
        post 
    from
        Post post   
    left join
        
    fetch
        post.user as user 
    where
        post.category = ?1 
    order by
        post.createdAt desc 
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 select
            p1_0.id,
            p1_0.category,
            p1_0.contents,
            p1_0.created_at,
            p1_0.modified_at,
            p1_0.district,
            p1_0.latitude,
            p1_0.longitude,
            u1_0.id,
            u1_0.city,
            u1_0.district,
            u1_0.email,
            u1_0.kakao_id,
            u1_0.nickname,
            u1_0.password,
            u1_0.user_rank,
            u1_0.role 
        from
            post p1_0 
        left join
            users u1_0 
                on u1_0.id=p1_0.user_id 
        where
            p1_0.category=? 
        order by
            p1_0.created_at desc 
        limit
            ?, ?

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    select
        pi1_0.id,
        pi1_0.file_name,
        pi1_0.img_url,
        pi1_0.user_id 
    from
        profile_image pi1_0 
    where
        pi1_0.user_id=?

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    select
        bm1_0.user_id,
        bm1_0.id,
        bm1_0.created_at,
        bm1_0.post_id,
        bm1_0.status 
    from
        book_mark bm1_0 
    where
        bm1_0.user_id=?
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    select
        i1_0.post_id,
        i1_0.id,
        i1_0.file_name,
        i1_0.img_url 
    from
        post_image i1_0 
    where
        i1_0.post_id=?
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    select
        c1_0.post_id,
        c1_0.id,
        c1_0.contents,
        c1_0.created_at,
        c1_0.modified_at,
        c1_0.parent_id,
        c1_0.user_id 
    from
        comment c1_0 
    where
        c1_0.post_id=?
Hibernate: 
    /* <criteria> */ 
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select
        count(l1_0.id) 
    from
        likes l1_0 
    left join
        post p1_0 
            on p1_0.id=l1_0.post_id 
    where
        p1_0.id=? 
        and l1_0.like_category_enum=?
Hibernate: 
    /* <criteria> */ 

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select
        count(l1_0.id) 
    from
        likes l1_0 
    left join
        post p1_0 
            on p1_0.id=l1_0.post_id 
    where
        p1_0.id=? 
        and l1_0.like_category_enum=?
Hibernate: 
    /* <criteria> */
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 select
        pi1_0.id,
        pi1_0.file_name,
        pi1_0.img_url,
        pi1_0.user_id 
    from
        profile_image pi1_0 
    left join
        users u1_0 
            on u1_0.id=pi1_0.user_id 
    where
        u1_0.id=?

2. left join

left join 은 게시물의 모든 데이터를 가져오며, 연결된 user 데이터가 있으면 이 데이터도 함께 가져온다. 그러나 이 쿼리에서는 user 엔티티의 데이터를 즉시 로드하지 않는다. 즉 user 데이터에 접근하려고 할 때, 필요에 따라 추가적인 쿼리가 실행될 수 있다.

select
        post 
    from
        Post post   
    left join
        post.user as user 
    where
        post.category = ?1 
    order by
        post.createdAt desc  
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select
            p1_0.id,
            p1_0.category,
            p1_0.contents,
            p1_0.created_at,
            p1_0.modified_at,
            p1_0.district,
            p1_0.latitude,
            p1_0.longitude,
            p1_0.user_id 
        from
            post p1_0 
        where
            p1_0.category=? 
        order by
            p1_0.created_at desc 
        limit
            ?, ?


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    select
        u1_0.id,
        u1_0.city,
        u1_0.district,
        u1_0.email,
        u1_0.kakao_id,
        u1_0.nickname,
        u1_0.password,
        u1_0.user_rank,
        u1_0.role,
        bm1_0.user_id,
        bm1_0.id,
        bm1_0.created_at,
        bm1_0.post_id,
        bm1_0.status 
    from
        users u1_0 
    left join
        book_mark bm1_0 
            on u1_0.id=bm1_0.user_id 
    where
        u1_0.id=?

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    select
        pi1_0.id,
        pi1_0.file_name,
        pi1_0.img_url,
        pi1_0.user_id 
    from
        profile_image pi1_0 
    where
        pi1_0.user_id=?

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    select
        i1_0.post_id,
        i1_0.id,
        i1_0.file_name,
        i1_0.img_url 
    from
        post_image i1_0 
    where
        i1_0.post_id=?
 
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    select
        c1_0.post_id,
        c1_0.id,
        c1_0.contents,
        c1_0.created_at,
        c1_0.modified_at,
        c1_0.parent_id,
        c1_0.user_id 
    from
        comment c1_0 
    where
        c1_0.post_id=?
Hibernate: 
    /* <criteria> */ 
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select
        count(l1_0.id) 
    from
        likes l1_0 
    left join
        post p1_0 
            on p1_0.id=l1_0.post_id 
    where
        p1_0.id=? 
        and l1_0.like_category_enum=?
Hibernate: 
    /* <criteria> */
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 select
        count(l1_0.id) 
    from
        likes l1_0 
    left join
        post p1_0 
            on p1_0.id=l1_0.post_id 
    where
        p1_0.id=? 
        and l1_0.like_category_enum=?
Hibernate: 
    /* <criteria> */
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 select
        pi1_0.id,
        pi1_0.file_name,
        pi1_0.img_url,
        pi1_0.user_id 
    from
        profile_image pi1_0 
    left join
        users u1_0 
            on u1_0.id=pi1_0.user_id 
    where
        u1_0.id=?

3. 사용 x

select
        post 
    from
        Post post 
    where
        post.category = ?1 
    order by
        post.createdAt desc */ 
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select
            p1_0.id,
            p1_0.category,
            p1_0.contents,
            p1_0.created_at,
            p1_0.modified_at,
            p1_0.district,
            p1_0.latitude,
            p1_0.longitude,
            p1_0.user_id 
        from
            post p1_0 
        where
            p1_0.category=? 
        order by
            p1_0.created_at desc 
        limit
            ?, ?
Hibernate: 
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    select
        u1_0.id,
        u1_0.city,
        u1_0.district,
        u1_0.email,
        u1_0.kakao_id,
        u1_0.nickname,
        u1_0.password,
        u1_0.user_rank,
        u1_0.role,
        bm1_0.user_id,
        bm1_0.id,
        bm1_0.created_at,
        bm1_0.post_id,
        bm1_0.status 
    from
        users u1_0 
    left join
        book_mark bm1_0 
            on u1_0.id=bm1_0.user_id 
    where
        u1_0.id=?
Hibernate: 

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    select
        pi1_0.id,
        pi1_0.file_name,
        pi1_0.img_url,
        pi1_0.user_id 
    from
        profile_image pi1_0 
    where
        pi1_0.user_id=?
Hibernate: 
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    select
        i1_0.post_id,
        i1_0.id,
        i1_0.file_name,
        i1_0.img_url 
    from
        post_image i1_0 
    where
        i1_0.post_id=?
Hibernate: 
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    select
        c1_0.post_id,
        c1_0.id,
        c1_0.contents,
        c1_0.created_at,
        c1_0.modified_at,
        c1_0.parent_id,
        c1_0.user_id 
    from
        comment c1_0 
    where
        c1_0.post_id=?
Hibernate: 
    /* <criteria> */ 
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select
        count(l1_0.id) 
    from
        likes l1_0 
    left join
        post p1_0 
            on p1_0.id=l1_0.post_id 
    where
        p1_0.id=? 
        and l1_0.like_category_enum=?
Hibernate: 
    /* <criteria> */ 
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select
        count(l1_0.id) 
    from
        likes l1_0 
    left join
        post p1_0 
            on p1_0.id=l1_0.post_id 
    where
        p1_0.id=? 
        and l1_0.like_category_enum=?
Hibernate: 
    /* <criteria> */ 

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select
        pi1_0.id,
        pi1_0.file_name,
        pi1_0.img_url,
        pi1_0.user_id 
    from
        profile_image pi1_0 
    left join
        users u1_0 
            on u1_0.id=pi1_0.user_id 
    where
        u1_0.id=?