性能会提升。
如一对多中，person对phone，会生成一张中间表person_phone
我有139，156，159，187的号码.现在要删除187的号码。过程大致如下：

delete from person_phone where id =1;
insert 139 into person_phone;
insert 156 into person_phone;
insert 159 into person_phone;
delete from phone where id=187;

这样性能低下。

加了mappedby，直接由多的一方维护，直接可以

delete from phone where id=187;