了解映射通过休眠中的注释

我试图理解JPA中注解的属性。我创建了以下示例，其中客户具有订单列表：mappedBy@OneToMany

@Entity
public class Customer {
   @Id @GeneratedValue public Integer getId() { return id; }
   public void setId(Integer id) { this.id = id; }
   private Integer id;

   @OneToMany(mappedBy="customer")
   @OrderColumn(name="orders_index")
   public List<Order> getOrders() { return orders; }
   public void setOrders(List<Order> orders) { this.orders = orders; }
   private List<Order> orders;
}

@Entity
@Table(name="TBL_ORDER")
public class Order {
   @Id @GeneratedValue public Integer getId() { return id; }
   public void setId(Integer id) { this.id = id; }
   private Integer id;

   public int getOrderNumber() { return orderNumber; }
   public void setOrderNumber(int orderNumber) { this.orderNumber = orderNumber; }
   private int orderNumber;

   @ManyToOne
   public Customer getCustomer() { return customer; }
   public void setCustomer(Customer customer) { this.customer = customer; }
   private Customer customer;
}

现在，当我使用Hibernate生成表时，我看到Hibernate只创建了2个表：

Hibernate: create table Customer (id number(10,0) not null, primary key (id))
Hibernate: create table TBL_ORDER (id number(10,0) not null, orderNumber number(10,0) not null, customer_id number(10,0), orders_index number(10,0), primary key (id))
Hibernate: alter table TBL_ORDER add constraint FK_nt24krtgqwcsynosqgk4jkvfv foreign key (customer_id) references Customer

另外，如果我尝试保存客户和一些订单，我会在下面看到Hibernate生成的DML语句：

Hibernate: insert into Customer (id) values (?)
Hibernate: insert into TBL_ORDER (customer_id, orderNumber, id) values (?, ?, ?)
Hibernate: update TBL_ORDER set orders_index=? where id=?

为什么休眠尝试在TBL_ORDER中插入和更新记录，而不仅仅是运行单个插入查询？

现在，如果我删除mappedBy属性并尝试生成表，这次我看到3个表：

Hibernate: create table Customer (id number(10,0) not null, primary key (id))
Hibernate: create table Customer_TBL_ORDER (Customer_id number(10,0) not null, orders_id number(10,0) not null, orders_index number(10,0) not null, primary key (Customer_id, orders_index))
Hibernate: create table TBL_ORDER (id number(10,0) not null, orderNumber number(10,0) not null, customer_id number(10,0), primary key (id))
Hibernate: alter table Customer_TBL_ORDER add constraint UK_sw94jktvh72tripj876s31052  unique (orders_id)
Hibernate: alter table Customer_TBL_ORDER add constraint FK_sw94jktvh72tripj876s31052 foreign key (orders_id) references TBL_ORDER
Hibernate: alter table Customer_TBL_ORDER add constraint FK_f03up2h945cg0dcbo2pdb1d3c foreign key (Customer_id) references Customer
Hibernate: alter table TBL_ORDER add constraint FK_nt24krtgqwcsynosqgk4jkvfv foreign key (customer_id) references Customer

为什么在这种情况下，休眠会创建一个额外的表？mappedBy 属性如何控制这一点？为什么我们对表中orders_id列有额外的唯一键约束Customer_TBL_ORDER？

现在，如果我尝试保存客户及其订单，我会得到以下DML操作：

Hibernate: insert into Customer (id) values (?)
Hibernate: insert into TBL_ORDER (customer_id, orderNumber, id) values (?, ?, ?)
Hibernate: insert into Customer_TBL_ORDER (Customer_id, orders_index, orders_id) values (?, ?, ?)

为什么在这种情况下，与我声明mappedBy属性的情况相比，我们没有额外的更新查询？