在FK上的JPA2 critera查询in-expression发出n个select语句,其中n是#values

Question

我对使用针对DB2和MySQL的hibernate有同样的问题.

这是一个测试:

        EntityManager em = emf.createEntityManager();
        CriteriaBuilder cb = em.getCriteriaBuilder();
        CriteriaQuery<Customers> query = cb.createQuery(Customers.class);
        Root<Customers> root = query.from(Customers.class);

        ArrayList<String> strList = new ArrayList<String>();
        strList.add("ADMIN");
        strList.add("SYSADMIN");
        strList.add("SALES");

        ArrayList<Predicate> predicateList = new ArrayList<Predicate>();

        Path<Groups> groups = root.get(Customers_.groups);
        Path<String> groupName = groups.get(Groups_.name);
        In<String> in = cb.in(groupName);
        for (String s : strList) { //has a value
            in = in.value(s);
        }
        predicateList.add(in);
        Predicate[] predicates = new Predicate[predicateList.size()];

        query.where(predicateList.toArray(predicates));
        TypedQuery<Customers> typedQuery = em.createQuery(query);
        this.outList = typedQuery.getResultList();

生成我需要的查询,然后是三个不需要的查询(strList中有多个值的附加查询).以下内容打印在日志中(我构建了第一个查询以将其分开.)第一个查询正是我想要的是接下来的三个查询,它们在生产中导致不需要的IO,我将其视为异常.请注意,如果in表达式不在FK上,则不会发生此问题.

INFO: Hibernate: 
select 
  customers0_.id as id0_, customers0_.fname as fname0_, customers0_.groups as groups0_, customers0_.lname as lname0_ 
from 
  test.customers customers0_ 
where 
  customers0_.groups in (? , ? , ?)

INFO: Hibernate: select groups0_.name as name1_0_ from test.groups groups0_ where groups0_.name=?
INFO: Hibernate: select groups0_.name as name1_0_ from test.groups groups0_ where groups0_.name=?
INFO: Hibernate: select groups0_.name as name1_0_ from test.groups groups0_ where groups0_.name=?

为什么额外三个查询？我该如何预防？我需要答案作为标准查询.

---

这是实体对象:

@Entity
@Table(name = "customers", catalog = "test", schema = "")
public class Customers implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "id", nullable = false)
    private Integer id;
    @Column(name = "fname", length = 45)
    private String fname;
    @Column(name = "lname", length = 45)
    private String lname;
    @JoinColumn(name = "groups", referencedColumnName = "name")
    @ManyToOne
    private Groups groups;

...getters and setters... 
}

下一个实体

@Entity
@Table(name = "groups", catalog = "test", schema = "")
public class Groups implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @Basic(optional = false)
    @Column(name = "name", nullable = false, length = 45, unique = true)
    private String name;
    @OneToMany(mappedBy = "groups")
    private Collection<Customers> customersCollection;

...getters and setters... 
}

编辑 ~~~~~~~~~~~~~~~~~~~~~~~~解决方案~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~:

将第二行添加到上面的代码可以解决问题(谢谢Clement):

Root<Customers> root = query.from(Customers.class);
root.fetch(Customers_.groups, JoinType.LEFT); //that's it that's all now it will not create the extra queries

Answer 1

进行附加查询的原因是您正在使用字段：private Groups groups;用于从客户到组的关联。由于 hibernate 无法拦截直接字段访问，因此当您获取 Customer 时，它必须获取 Groups 对象。它以 N+1 选择方式对条件查询执行此操作（而不是例如计算出来并自动执行联接或子选择）。

要解决此问题，您也可以告诉 hibernate 获取关联：

root.fetch(Customers_.groups, JoinType.LEFT); // LEFT join since your schema could have a customer with a null group.