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JPA @OneToOne 避免 N+1 SELECT

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csongyu/jpa-fetch

branch: feature/@OneToOne

N+1 SELECT

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@Entity
@Data
@EqualsAndHashCode(of = {"code"})
public class Employee implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    private String name;

    @Column(nullable = false, unique = true)
    private String code;

    // whatever FetchType is LAZY or EAGER
    @OneToOne(mappedBy = "employee", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private Address address;
}

@Entity
@Data
@ToString(exclude = {"employee"})
@EqualsAndHashCode(of = {"id"})
public class Address {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    private String city;

    private String street;

    @OneToOne
    private Employee employee;
}
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public interface EmployeeRepository extends JpaRepository<Employee, Long> {
    Set<Employee> findByCodeIn(Set<String> codes);
}
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// employeeCodes size is 3, 3+1 SELECTs in total
final Set<Employee> employees = this.repository.findByCodeIn(employeeCodes);
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Hibernate: 
    select
        employee0_.id as id1_1_,
        employee0_.code as code2_1_,
        employee0_.name as name3_1_ 
    from
        employee employee0_ 
    where
        employee0_.code in (
            ? , ? , ?
        )
Hibernate: 
    select
        address0_.id as id1_0_1_,
        address0_.city as city2_0_1_,
        address0_.employee_id as employee4_0_1_,
        address0_.street as street3_0_1_,
        employee1_.id as id1_1_0_,
        employee1_.code as code2_1_0_,
        employee1_.name as name3_1_0_ 
    from
        address address0_ 
    left outer join
        employee employee1_ 
            on address0_.employee_id=employee1_.id 
    where
        address0_.employee_id=?
Hibernate: 
    select
        address0_.id as id1_0_1_,
        address0_.city as city2_0_1_,
        address0_.employee_id as employee4_0_1_,
        address0_.street as street3_0_1_,
        employee1_.id as id1_1_0_,
        employee1_.code as code2_1_0_,
        employee1_.name as name3_1_0_ 
    from
        address address0_ 
    left outer join
        employee employee1_ 
            on address0_.employee_id=employee1_.id 
    where
        address0_.employee_id=?
Hibernate: 
    select
        address0_.id as id1_0_1_,
        address0_.city as city2_0_1_,
        address0_.employee_id as employee4_0_1_,
        address0_.street as street3_0_1_,
        employee1_.id as id1_1_0_,
        employee1_.code as code2_1_0_,
        employee1_.name as name3_1_0_ 
    from
        address address0_ 
    left outer join
        employee employee1_ 
            on address0_.employee_id=employee1_.id 
    where
        address0_.employee_id=?

JOIN FETCH

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public interface EmployeeRepository extends JpaRepository<Employee, Long> {
    @Query("from Employee e join fetch e.address where e.code in :code")
    Set<Employee> queryByCodeIn(@Param("code") Set<String> codes);
}
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// employeeCodes size is 3, only 1 SELECT in total
final Set<Employee> employees = this.repository.queryByCodeIn(employeeCodes);
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Hibernate: 
    select
        employee0_.id as id1_1_0_,
        address1_.id as id1_0_1_,
        employee0_.code as code2_1_0_,
        employee0_.name as name3_1_0_,
        address1_.city as city2_0_1_,
        address1_.employee_id as employee4_0_1_,
        address1_.street as street3_0_1_ 
    from
        employee employee0_ 
    inner join
        address address1_ 
            on employee0_.id=address1_.employee_id 
    where
        employee0_.code in (
            ? , ? , ?
        )