性能休眠@Subselect与数据库视图

如何解决性能休眠@Subselect与数据库视图

我有一个基于Java 8 Spring Boot 2.3.3的应用程序（使用休眠5.4.20），下面有一个Postgresql。 我想最后了解一下（对于性能而言）是否使用数据库视图和@Subselect更好。

快速浏览一下：我有一个实体“ Book”和3个实体“ BookRank”（用户给这些书1至10个 stars ），“ BookComment”（用户对该书的评论） ，“ BookLike”（用户将 like 放入书中）每个都具有Book关系（ManyToOne），因此在查询结果中，我想按以下方式订购书：

order by avg(book_rank) desc,sum(book_rank) desc,count(book_comment) desc,count(book_like) desc

当然有很多书，有很多排名，评论和喜欢...：）

我发现了从复杂的查询开始的4种方法，其中包含3个子选择（这对我来说是最好的解决方案，但是如果有人有更好的方法，请告诉我）。 / em>

select *
from(
    select bbb.id as book_id,min(bbb.b_rank_average) as b_rank_average,min(bbb.b_rank_sum) as b_rank_sum,min(bbb.b_comment_count) as b_comment_count,count(b_l.id) as b_like_count
    from(
        select rr.id,min(bb.b_rank_average) as b_rank_average,min(bb.b_rank_sum) as b_rank_sum,count(b_c.id) as b_comment_count
        from(           
            select b.id,avg(b_r.rank) as b_rank_average,sum(b_r.rank) as b_rank_sum
            from book as b
            left join book_rank as b_r on (b.id = b_r.book_id and b_r.deleted = false)
            group by b.id) as bb
            
        left join book_comment as b_c on (bb.id = b_c.book_id and b_c.deleted = false)
        group by bb.id) as bbb
                
    left join book_like as b_l on (bbb.id = b_l.book_id and b_l.deleted = false)
    group by bbb.id
    ) as bbbb
    
left join book on bbbb.book_id = book.id
where book.deleted = false
order by b_rank_average desc nulls last,b_rank_sum desc nulls last,b_comment_count desc,b_like_count desc,book_id desc;

我想知道哪个是性能最好的...

1）存储库中的纯本机查询-无视图-无不可变实体

在我的存储库中，我只有一个方法，上面编写了本机查询

2）在存储库中具有数据库视图和本机查询

使用查询的中心部分创建数据库视图：

create or replace view book_ranking as

    select bbb.id as book_id,sum(b_r.rank) as b_rank_sum
            from book as b
            left join book_rank as b_r on (b.id = b_r.book_id and b_r.deleted = false)
            group by b.id) as bb
            
        left join book_comment as b_c on (bb.id = b_c.book_id and b_c.deleted = false)
        group by bb.id) as bbb
                
    left join book_like as b_l on (bbb.id = b_l.book_id and b_l.deleted = false)
    group by bbb.id

并修改存储库方法中的本机查询以仅使用视图：

select * 
from book_ranking as the_view
left join book on the_view.book_id = book.id 
where book.deleted = false
order by b_rank_average desc nulls last,book_id desc;

3）具有数据库视图和不可变实体，该实体在存储库中选择视图和jpql查询

使用实体（不可变）封装上方的视图

@Entity
@Subselect("select * from book_ranking")
public class BookRanking implements Serializable {
    ....
}

并修改连接Book实体和新的BookRanking不可变实体（即视图）的存储库查询（非本机）

@Query("select b from BookRanking as b_r 
        join Book b on b_r.bookId = b.id 
        where b.deleted = false
        order by b_r.b_rank_average desc nulls last,b_r.b_rank_sum desc nulls last,b_r.b_comment_count desc,b_r.b_like_count desc,b_r.bookId desc)

4）在存储库中@subselect和jpql查询中没有查询，但视图是不变的

在数据库上没有任何视图，但不可变实体BookRanking在@Subselect批注中具有带有“视图查询”和@Synchronize批注的查询，如下所示：

@Entity
@Subselect("select bbb.id as book_id,sum(b_r.rank) as b_rank_sum
            from book as b
            left join book_rank as b_r on (b.id = b_r.book_id and b_r.deleted = false)
            group by b.id) as bb
            
        left join book_comment as b_c on (bb.id = b_c.book_id and b_c.deleted = false)
        group by bb.id) as bbb
                
    left join book_like as b_l on (bbb.id = b_l.book_id and b_l.deleted = false)
    group by bbb.id )

@Synchronize({ "book","book_rank","book_comment","book_like" })

public class BookRanking implements Serializable {
    ....
}

，并使用连接第3点的Book实体和新的BookRanking不可变实体（如视图）的相同存储库查询（非本机）

解决方法

通常，数据库视图只是通常在解析期间扩展为查询计划的关系，因此使用视图与直接写出整个查询应该没有显着差异。使用视图可以使您更轻松地重用查询，但是当您要更改某些内容时，您将不得不更改视图，也可能需要更改所有使用该视图的应用程序，因此可重用性方面会让您望而却步。

我通常不推荐视图，因为我看到人们出于“使他人更容易”的唯一目的而在视图中加入了许多不必要的联接。未使用的联接的问题在于，数据库通常无法消除它们。 TLDR，我建议直接在代码中写出查询，因为您可以省略不需要的联接，从而获得更好的性能。

您可以使用以下更简单的查询：

select 
    b.id,avg(b_r.rank) as b_rank_average,sum(b_r.rank) as b_rank_sum,(select count(*) from book_comment as b_c where b.id = b_c.book_id and b_c.deleted = false) as b_comment_count
    (select count(*) from book_like as b_l where b.id = b_l.book_id and b_l.deleted = false) as b_like_count
from book as b
left join book_rank as b_r      on (b.id = b_r.book_id and b_r.deleted = false)
where b.deleted = false
group by b.id
order by b_rank_average desc nulls last,b_rank_sum desc nulls last,b_comment_count desc,b_like_count desc,b.id desc

也可以用JPQL / HQL查询建模。看起来非常相似：

select 
    b.id,avg(r.rank) as b_rank_average,sum(r.rank) as b_rank_sum,(select count(*) from b.comments c where c.deleted = false) as b_comment_count
    (select count(*) from b.booksLike l where l.deleted = false) as b_like_count
from book as b
left join b.ranks as r on r.deleted = false
where b.deleted = false
group by b.id
order by b_rank_average desc nulls last,b.id desc