Hibernate级联删除的疑问

今天做了个测试，一共两张表，person和order，person是one方，order是many方。
person的hbm文件内容如下：
<hibernate-mapping package="cn.itcast.bean">
  <class name="Person" table="person">
    <id name="id" column="id" type="java.lang.Integer">
      <generator class="native"/>
    </id>
   
    <property name="name" column="name" type="java.lang.String" not-null="true"/>
   
    <set name="order" table="Order" outer-join="auto" cascade="delete" lazy="false">
    <key column="person_id"/>
    <one-to-many class="cn.itcast.bean.Order"/>
    </set>
  </class>
</hibernate-mapping>

order的hbm文件内容如下：
<hibernate-mapping package="cn.itcast.bean">
  <class name="Order" table="orders">
    <id name="id" column="id" type="java.lang.Integer">
      <generator class="native"/>
    </id>
   
    <property name="personId" column="person_id" type="java.lang.Integer" not-null="true"/>
    <property name="name" column="name" type="java.lang.String" not-null="true"/>
  </class>
</hibernate-mapping>

删除时我是调用的personBean的一个delete方法去删除一条person记录的，然后我也想级联删除掉order表的记录，MyEclipse控制台打印出的语句如下(此次测试有3条order表的关联记录)：
21:03:17,203 INFO  [STDOUT] Hibernate: select person0_.id as id0_0_, person0_.name as name0_0_ from person person0_ where person0_.id=?
21:03:17,218 INFO  [STDOUT] Hibernate: select order0_.person_id as person2_1_, order0_.id as id1_, order0_.id as id1_0_, order0_.person_id as person2_1_0_, order0_.name as name1_0_ from orders order0_ where order0_.person_id=?
21:03:17,218 INFO  [STDOUT] Hibernate: update orders set person_id=null where person_id=?
21:03:17,218 INFO  [STDOUT] Hibernate: delete from orders where id=?
21:03:17,218 INFO  [STDOUT] Hibernate: delete from orders where id=?
21:03:17,218 INFO  [STDOUT] Hibernate: delete from orders where id=?
21:03:17,218 INFO  [STDOUT] Hibernate: delete from person where id=?

不解的问题如下：
1. 级联删除时，为什么hibernate要先把many方的关联的外键update成null而不是直接一条delete语句啊？该语句如下：update orders set person_id=null where person_id=?，因为我一开始建数据库时把外键设置成not-null，所以在执行到这里的时候控制台抛异常了，后来我把外键的not-null限制去掉才可以正确执行。
2. 为什么要把many方关联的记录先select出来再一条一条的delete呢？这个太影响效率了吧？还是我的hbm文件配置上有不合理的地方需要改进？
请大侠们指点！