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 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

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Tags: 映射映射 约束约束
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