JPQL 查询导致多次往返数据库甚至 EAGER 和 JOIN FETCH

Question

我试图理解为什么我的查询会导致 2 次调用数据库。 据我了解，我在查询中使用 FETCH 关键字进行了 EAGER 加载，这应该会导致一次往返，但在下面情况并非如此。感谢任何提示！

        TypedQuery<Recipe> query = em.createQuery("SELECT r FROM Recipe r" +
                "  LEFT JOIN FETCH r.ingredients ri LEFT JOIN FETCH r.author a WHERE r.id= :id ", Recipe.class);
        
        query.setParameter("id", id);

配方Class：

@Entity
@Table(name = "recipes")
@Getter
@Setter
@NoArgsConstructor
@JsonIdentityInfo(
        generator = ObjectIdGenerators.PropertyGenerator.class,
        property = "id")
@JsonSerialize(using = RecipeMetaSerializer.class)
public class Recipe implements Serializable {

    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    private Long id;
    
    
    @ManyToOne(fetch = FetchType.EAGER)
    private User author;


    @OneToMany(
            mappedBy = "recipe",
            orphanRemoval = true,
            fetch = FetchType.LAZY,
            cascade = CascadeType.PERSIST
    )
    private List<RecipeIngredient> ingredients;
}

首先加入表 RecipeIngredient：

@Entity
@Table(name="recipe_ingredients")
@Getter
@Setter
@NoArgsConstructor
@IdClass(RecipeIngredientId.class)
public class RecipeIngredient implements Serializable {
    
    @Id
    @ManyToOne(fetch= FetchType.EAGER)
    private Recipe recipe;

    @Id
    @ManyToOne(fetch= FetchType.LAZY)
    private Ingredient ingredient;
.....
}

第二个连接表：

@Entity
@Table(name = "users")
@Getter
@Setter
@NoArgsConstructor
public class User {
    
    @Id
    private Long id;
    
    @OneToMany(fetch = FetchType.LAZY, mappedBy = "author")
    private List<Recipe> recipes;
    
}

JPQL 查询导致以下两次对 DB 的调用都包含对表 users 的左外连接：

select recipe0_.id as id1_6_0_, ingredient1_.ingredient_id as ingredie4_5_0__, user2_.img_url as img_url2_7_2_, user2_.username as username4_7_2_ from recipes recipe0_ **left outer join recipe_ingredients** ingredient1_ on recipe0_.id=ingredient1_.recipe_id **left outer join users** user2_ on recipe0_.author_id=user2_.id where recipe0_.id=?

select recipe0_.id as id1_6_0_, user1_.username as username4_7_1_ from recipes recipe0_ **left outer join users** user1_ on recipe0_.author_id=user1_.id where recipe0_.id=?

我希望加入 Users 表一次，而不是两次。有什么想法吗？ 谢谢！

Answer 1

看起来第二个查询是这里的Recipe recipe

@Entity
public class RecipeIngredient {
    
    @Id
    @ManyToOne(fetch= FetchType.EAGER)
    private Recipe recipe;
 
}

只需使用FetchType.LAZY

@Entity
public class RecipeIngredient {
    
    @Id
    @ManyToOne(fetch= FetchType.LAZY)
    private Recipe recipe;
 
}

如果您使用entityManager.find()方法，则不会有第二个查询。 recipe将已经在缓存中。

但是对于 JPQL Hibernate 认为在第一个查询中找到的recipe不在缓存中，所以它再次获取它（即使它是相同的recipe ）。

建议

始终在任何地方使用延迟加载。 在运行时禁用预加载是不可能的。 此外，如果您想将eager更改为lazy ，将很难测试所有内容。

https://vladmihalcea.com/eager-fetching-is-a-code-smell