实现了一对多关系,并且工作正常。
我的问题是当我运行以下查询时,如果表中有100个员工行,并且每个员工都有2个部门。数据库查询被调用101次,因为对于每个员工来说,它都在呼叫部门查询,所以要花很长时间才能完成所有一百行的调用,请问有人可以提出其他解决方案吗?
请查看下面的详细信息
它正在调用的查询:
First query is : SELECT * FROM Employee e
Next 100 queries : SELECT * FROM DEPARTMENT d WHERE d.EmployeeId=?
JPA数据库调用:
javax.persistence.Query query = em.createNamedQuery("SELECT * FROM Employee e", Employee.class);
return query.getResultList();
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.Id;
import javax.persistence.NamedNativeQueries;
import javax.persistence.NamedNativeQuery;
import javax.persistence.OneToMany;
import javax.persistence.Table;
@Entity
@Table(name = "EMPLOYEE")
public class Employee implements Serializable
{
@Id
@Column(name = "EmployeeId")
String employeeId;
@OneToMany(mappedBy = "employee", cascade = CascadeType.ALL, fetch = FetchType.EAGER)
private List<Department> departments;
public List<Department> getDepartments() {
return departments;
}
public void setDepartments(List<Department> departments) {
this.departments = departments;
}
public String getEmployeeId() {
return employeeId;
}
public void setEmployeeId(String employeeId) {
this.employeeId = employeeId;
}
}
@Entity
@Table(name = "DEPARTMENT")
public class Department implements Serializable
{
private static final long serialVersionUID = 1L;
@Id
@Column(name = "DepartmentID")
String departmentId;
@ManyToOne(fetch = FetchType.EAGER)
@JoinColumn(name = "EmployeeId", insertable = false, updatable = false)
private Employee employee;
}
输出xml:
<Employees>
<Employee>
<name>Rob</name>
<Departments>
<Departmnet><id>1</id></Departmnet>
<Departmnet><id>2</id></Departmnet>
</Departments>
</Employee>
<Employee>
<name>Sam</name>
<Departments>
<Departmnet><id>1</id></Departmnet>
<Departmnet><id>2</id></Departmnet>
</Departments>
</Employee>
</Employees>
参考方案
这是典型的N+1 selects issue。我通常用described here和here的JOIN FETCH
查询解决此问题
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