[疑难杂症]hibernate annotation多对多问题：insert关系前先delete

我用hbiernate annation写的代码，维护多对多关系时，出了个奇怪的问题：

1：Operator.java
[code]

@Entity
//@Table(name="operator")
public class Operator implements java.io.Serializable,IfTableModel {

private static final long serialVersionUID = 1L;
public static final String [] column={"编号","姓名","其他信息"};

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "id")
private Integer id;
@Column(name = "name",unique=true)
private String name;
@Column(name = "msg")
private String msg;

@ManyToOne(targetEntity=base.Role.class)

@JoinColumn(name="RoleId")
private Role role;

@ManyToMany(targetEntity=base.Department.class)
@Cascade({org.hibernate.annotations.CascadeType.SAVE_UPDATE})
@JoinTable(
name="operator_department ",
joinColumns={@JoinColumn(name="OperatorId")},
inverseJoinColumns={@JoinColumn(name="DepartId")}
)
private List<Department> departs=new ArrayList();


}
[/code]

2:Department.java
[code]

@Entity
@Table(name = "department")

public class Department implements java.io.Serializable,IfTableModel {

private static final long serialVersionUID = 1L;
public static final String Id = "id";
public static final String Name = "name";
public static final String [] column={"编号","部门名称"};

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "id")
private Integer id;
@Column(name = "name")
private String name;
@ManyToMany(targetEntity=base.Operator.class,mappedBy="departs")
private List<Operator> operators=new ArrayList();

}
[/code]

关系说明：operator和department是多对多的关系，其中，operator端负责关系的维护
3:对operator和department操作的代码
[code]
Transaction tx2= se.beginTransaction();
BaseUtil u=new BaseUtil();
List<Department> departs=u.getDepartments();
List<Operator> opers=u.getOperators();
opers.get(0).getDeparts().add(departs.get(0));
opers.get(0).getDeparts().add(departs.get(1));
tx2.commit();
[/code]
现在期望的sql代码是：insert into operator_department (OperatorId, DepartId) values (?, ?)
这里出现一个奇怪的问题是，现在总是先
delete from operator_department where OperatorId=?
然后才把所有原来的关系重新插入一遍
insert into operator_department (OperatorId, DepartId) values (?, ?)
insert into operator_department (OperatorId, DepartId) values (?, ?)
insert into operator_department (OperatorId, DepartId) values (?, ?)
这样严重影响性能，各位高人帮忙看看什么原因啊？

奇怪的问题啊！！！