非主键的 JPA Hibernate FetchType.EAGER 和 FetchMode.JOIN

Question

我遇到了一个非常有趣的场景。我知道 n+1 问题以及 FetchType.EAGER 和 FetchMode.JOIN。 我有一个父实体学校，它有 2 个@OneToMany 子实体，即学生和教师。我需要全部 3 个，所以使用 FetchType.EAGER 和 FetchMode.JOIN。

学校实体

import lombok.AllArgsConstructor;
import lombok.Getter;
import lombok.NoArgsConstructor;
import org.hibernate.annotations.Fetch;
import org.hibernate.annotations.FetchMode;
import javax.persistence.*;
import java.util.Set;
@Entity
@Getter
@NoArgsConstructor
@AllArgsConstructor
public class School {
    @Id
    @GeneratedValue(generator = "sequence", strategy = GenerationType.IDENTITY)
    @SequenceGenerator(name = "sequence", allocationSize = 10)
    int schoolId;
    String schoolName;
    float schoolRating;
    @OneToMany(mappedBy = "school", cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.EAGER)
    @Fetch(FetchMode.JOIN)
    private Set<Teacher> teachers;
    @OneToMany(mappedBy = "school", cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.EAGER)
    @Fetch(FetchMode.JOIN)
    private Set<Student> students;
 }

学生实体

import lombok.AllArgsConstructor;
import lombok.NoArgsConstructor;
import javax.persistence.*;
import java.util.Date;
@NoArgsConstructor
@AllArgsConstructor
@Entity
public class Student {
    @Id
    @GeneratedValue(generator = "sequence", strategy = GenerationType.IDENTITY)
    @SequenceGenerator(name = "sequence", allocationSize = 10)
    public int studentId;
    public byte studentByte;
    public Date date;
    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "schoolId", referencedColumnName = "schoolId")
    private School school;
}

教师实体

import lombok.AllArgsConstructor;
import lombok.NoArgsConstructor;    
import javax.persistence.*;
import java.util.Date;
@NoArgsConstructor
@AllArgsConstructor
@Entity
public class Teacher {
    @Id
    @GeneratedValue(generator = "sequence", strategy = GenerationType.IDENTITY)
    @SequenceGenerator(name = "sequence", allocationSize = 10)
    public int teacherId;
    public byte teacherByte;
    public Date date;
    @ManyToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinColumn(name = "schoolId", referencedColumnName = "schoolId")
    private School school;
}

学校回购

@Repository
public interface SchoolRepository extends JpaRepository<School, Integer> {
List<School>findBySchoolName(String schoolName);
}

如果我通过findById方法得到School对象，即表的主键。

Optional<School> schoolById = schoolRepository.findById(1);

生成的 SQL 是学校、学生和教师实体的连接。

select school0_.schoolId as schoolid1_0_0_, school0_.schoolName as schoolna2_0_0_, school0_.schoolRating as schoolra3_0_0_, students1_.schoolId as schoolid4_1_1_, students1_.studentId as studenti1_1_1_, students1_.studentId as studenti1_1_2_, students1_.date as date2_1_2_, students1_.schoolId as schoolid4_1_2_, students1_.studentByte as studentb3_1_2_, teachers2_.schoolId as schoolid4_2_3_, teachers2_.teacherId as teacheri1_2_3_, teachers2_.teacherId as teacheri1_2_4_, teachers2_.date as date2_2_4_, teachers2_.schoolId as schoolid4_2_4_, teachers2_.teacherByte as teacherb3_2_4_ from School school0_ left outer join Student students1_ on school0_.schoolId=students1_.schoolId left outer join Teacher teachers2_ on school0_.schoolId=teachers2_.schoolId where school0_.schoolId=?

但是如果我找到一所学校有其他一些不是主键的变量。

List<School> schoolByName = schoolRepository.findBySchoolName("school1");

生成的 SQL 是对 DB 的 3 个不同的命中。

Hibernate: select school0_.schoolId as schoolid1_0_, school0_.schoolName as schoolna2_0_, school0_.schoolRating as schoolra3_0_ from School school0_ where school0_.schoolName=?
Hibernate: select teachers0_.schoolId as schoolid4_2_0_, teachers0_.teacherId as teacheri1_2_0_, teachers0_.teacherId as teacheri1_2_1_, teachers0_.date as date2_2_1_, teachers0_.schoolId as schoolid4_2_1_, teachers0_.teacherByte as teacherb3_2_1_ from Teacher teachers0_ where teachers0_.schoolId=?
Hibernate: select students0_.schoolId as schoolid4_1_0_, students0_.studentId as studenti1_1_0_, students0_.studentId as studenti1_1_1_, students0_.date as date2_1_1_, students0_.schoolId as schoolid4_1_1_, students0_.studentByte as studentb3_1_1_ from Student students0_ where students0_.schoolId=?

我意识到只有在我们通过 id 即主键获取时加入才有效，但我没有 School 的主键。我有学校的名称，它是唯一的和索引的，也需要学生实体和教师实体。有没有办法让他们都在休眠中使用加入。我知道如果学生和老师的记录更多，那将是性能下降，但就我而言，最多只有 3-4 条记录。这就是为什么我想加入他们。

Answer 1

• 不建议使用FetchMode.JOIN 映射一个实体的多个关联集合字段。这是为了避免 Cartesian product issue。我很惊讶即使您选择了Id，它也执行了 sql 连接

• 当您获取除 Id 字段之外的 School 时，hibernate 不知道您将获取多少个 School，因此如果它执行连接获取而不是单独选择，它将以 Cartesian product issue

• 假设您有 10 所学校，每所学校有 20 名教师和 400 名学生。如果 hibernate 进行了连接，它将必须从 db 中带来 80,000 (10*20*400) 记录。 但是由于是单独选择，所以会带来4,210（10 + 200 + 4000）的记录。即使在由Id 选择的情况下，它也是420 记录与8000 记录

简答

即使您找到了一种方法，也不要使用连接检索父实体及其多个关联集合，因为性能会比多选差。

更新：

• 如果您确定学校名称是唯一的，并且每个学校只有几位老师并且学生人数很少，您可以执行以下操作：（当前您的findBySchoolName返回List<School>，您可以将其更改为返回可选学校）

@Repository
public interface SchoolRepository extends JpaRepository<School, Integer> {
    
    @Query("SELECT s from School s left join fetch s.teachers " +
            "left join fetch s.students where s.schoolName = :name")
    Optional<School> findBySchoolName(String name);
}