Question

我对hibernate有一个有趣的问题，该模型如下所示：

@NamedQueries({
    @NamedQuery(name = "A.test", query = "SELECT DISTINCT a FROM A a LEFT JOIN FETCH a.pk.b WHERE a.pk.b.pk.c.id = :cId AND a.pk.b.pk.d.id = :dId")
})
@Entity
@Table(name = "AT")
public class A implements Serializable {

    @EmbeddedId
    private APK pk;
}

@Embeddable
public class APK implements Serializable {

    @ManyToOne
    @JoinColumns({@JoinColumn(name = "***", referencedColumnName = "***"),
                  @JoinColumn(name = "***", referencedColumnName = "***")
    })
    private B b;
}

@Entity
@Table(name = "BT")
public class B implements Serializable {

    @EmbeddedId
    private BPK pk;
}


@Embeddable
public class APK implements Serializable {

    @ManyToOne
    @JoinColumn(name = "***", referencedColumnName = "***")
    private C c;

    @ManyToOne
    @JoinColumn(name = "***", referencedColumnName = "***")
    private D d;
}

问题是命名查询会产生额外的SQL查询...... 问题是什么？谢谢！

Answer 1

默认情况下，Hibernate会急切地提取多对一的引用。在命名查询中选择“A”时，会有多个一对一的引用将被解析，这些引用将与您看到的SQL查询相对应。 hibernate的默认行为可以通过在多对一注释上明智地放置延迟加载来覆盖，即：

@ManyToOne(fetch=FetchType.LAZY)

即使我使用Left join fetch，Hibernate也会生成多个SQL查询

1 个答案: