[转]Hibernate映射文件中索引及约束的使用
1、添加索引：在一对多的关系中，在多的一方会产生一个外键，这个外键没有自动添加索引，当存在从一的一端产生对多的一端的查询时，有可能会在多的一端造成全表查询问题，数据量巨大时会产生严重的性能问题。可以在多一端的外键上添加索引（index="user_group_id_idx"）来解决这个问题。例如：?
<?xml version="1.0"?>?
<!DOCTYPE hibernate-mapping PUBLIC?
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"?
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">?
<hibernate-mapping>?
<class name="com.bjsxt.hibernate.User" table="t_user">?
? <id name="id">?
?? <generator column="group_id" index="user_group_id_idx"></many-to-one>?
</class>?
</hibernate-mapping>?
发出的SQL语句为：?
create index user_group_id_idx on t_user (group_id)?
2、添加约束：id会产生主键约束，同时会建立索引；key会产生外键，但默认不会建立索引；在class或set上通过使用check关键字来产生check约束（例如：<set name="emailAddresses" table="t_email" check="email_address like '%@%'">）；单个字段的唯一键通过（unique="true"）设置，多个字段联合唯一通过在需要联合的字段上添加（unique-key="name"）来实现，其中对联合唯一索引的unique-key的值必须相同；通过not-null="true"来设置字段的非空约束。?
<?xml version="1.0"?>?
<!DOCTYPE hibernate-mapping PUBLIC?
??????? "-//Hibernate/Hibernate Mapping DTD 3.0//EN"?
??????? "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">?
<hibernate-mapping>?
<class name="events.Person" table="t_person">?
? <id name="id" column="person_id">?
?? <generator unique-key="name"></property>?
? <property name="lastName" unique-key="name"></property>?
? <set name="emailAddresses" table="t_email" check="email_address like '%@%'">?
?? <key column="person_id"></key>?
?? <element column="email_address" type="string"></element>?
? </set>?
? <set name="events" table="t_person_event">?
?? <key column="person_id"></key>?
?? <many-to-many column="event_id"></many-to-many>?
? </set>?
</class>?
</hibernate-mapping>?
产生的SQL语句是：?
??? drop table t_email cascade constraints?
??? drop table t_event cascade constraints?
??? drop table t_person cascade constraints?
??? drop table t_person_event cascade constraints?
??? drop sequence event_id_seq?
??? drop sequence person_id_seq?
??? create table t_email (?
??????? person_id number(19,0) not null,?
??????? email_address varchar2(255 char),?
??????? check (email_address like '%@%')?
??? )?
??? create table t_event (?
??????? event_id number(19,0) not null,?
??????? title varchar2(255 char),?
??????? event_date date,?
??????? primary key (event_id)?
??? )?
??? create table t_person (?
??????? person_id number(19,0) not null,?
??????? age number(10,0),?
??????? firstName varchar2(255 char),?
??????? lastName varchar2(255 char),?
??????? primary key (person_id),?
??????? unique (firstName, lastName)?
??? )?
??? create table t_person_event (?
??????? event_id number(19,0) not null,?
??????? person_id number(19,0) not null,?
??????? primary key (person_id, event_id)?
??? )?
??? alter table t_email?
??????? add constraint FKA03188117708282F?
??????? foreign key (person_id)?
??????? references t_person?
??? alter table t_person_event?
??????? add constraint FKC7F6A31B7708282F?
??????? foreign key (person_id)?
??????? references t_person?
??? alter table t_person_event?
??????? add constraint FKC7F6A31BF96D1A45?
??????? foreign key (event_id)?
??????? references t_event?
??? create sequence event_id_seq?
??? create sequence person_id_seq?
