hibernate - Hibernate 在 InheritanceType.JOINED 查询中不使用鉴别​​器

Question

我有以下实体：

@Entity
@Table(name = "employee")
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorColumn(name="TYPE") 
public class Employee {}

@Entity
@Table(name = "regular_employee")
@PrimaryKeyJoinColumn(name = "id")
@DiscriminatorValue("R")
public class RegularEmployee extends Employee{}

hibernate 在查询中不使用鉴别​​器值的问题。

select
    employee0_.id as id1_1_,
    employee0_.name as name2_1_,
    employee0_.type as type3_1_,
    employee0_1_.pay_per_hour as pay_per_1_0_,
    case 
        when employee0_1_.id is not null then 1 
        when employee0_.id is not null then 0 
    end as clazz_ 
from
    employee employee0_ 
left outer join
    regular_employee employee0_1_ 
        on employee0_.id=employee0_1_.id 

hibernate 不应该在“left jouter join”部分使用鉴别器值吗？就像是：

left outer join
    regular_employee employee0_1_ 
        on employee0_.type='R' and employee0_.id=employee0_1_.id

我猜这会带来性能的提升。

谢谢，泰金。

Answer 1

使用 JOINED 继承策略和左连接时，不需要鉴别器值来获取所需的数据。在这种情况下，JPA 规范并没有真正强制使用鉴别器——在Hibernate 4:persisting InheritanceType.JOINED discriminator column values的回答中解释了这一点。

也就是说，Hibernate 团队似乎很自豪他们不需要使用鉴别器值，这可以从评论HHH-6911中观察到。