Question

我有两个JPA实体

public class Job {
  @ManyToOne
  @JoinColumn(name = "service")
  public Service service;

  @Column(name = "queue_time")
  public Long queueTime;

  @Column(name = "run_time")
  public Long runTime;
}

public class Service {
  @Id
  @Column(name = "id")
  public Long id;

  @Column(name = "name")
  public String name;

  @Column(name = "host")
  public String host;
}

现在我想用JPQL进行一些聚合查询：

SELECT job.service.id, AVG(job.queueTime), AVG(job.runTime) FROM Job job GROUP BY job.service.id

生成的SQL查询（我使用的是MySQL数据库）如下所示：

SELECT t0.id, AVG(t1.queueTime), AVG(t1.runTime) FROM Service t0, Job t1 WHERE (t0.service = t1.id) GROUP BY t0.id

如您所见，JPA将我的JPQL查询转换为带有连接的SQL查询。然而，这会大大减慢查询速度。以下SQL查询执行速度提高约6倍，并返回完全相同的结果集：

SELECT t1.service, AVG(t1.queueTime), AVG(t1.runTime) FROM Job t1 GROUP BY t1.service

如果我将JPQL查询更改为

SELECT job.service, AVG(job.queueTime), AVG(job.runTime) FROM Job job GROUP BY job.service

生成的SQL查询如下所示：

SELECT t0.id, t0.name, t0.host AVG(t1.queueTime), AVG(t1.runTime) FROM Service t0, Job t1 WHERE (t0.service = t1.id) GROUP BY t0.id, t0.name, t0.host

有没有办法编写只用查询作业表而不连接服务表的JPQL？

Answer 1

这个问题为我解决了这个问题：How can I retrieve the foreign key from a JPA ManyToOne mapping without hitting the target table?

我采用了第二种解决方案（b）使用FK的只读字段）

使用JPQL查询JoinColumn的值

1 个答案: