Hibernate之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 class="sequence"> 
<param name="sequence">user_id_seq</param> 
</generator> 
</id> 
<property name="name"></property> 
<many-to-one name="group" 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 class="sequence"> 
<param name="sequence">person_id_seq</param> 
</generator> 
</id> 
<property name="age"></property> 
<property name="firstName" 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 class="events.Event" 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