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[ASP.net教程]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: 

<??> <!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> 

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发出的SQL语句为: 

create index user_group_id_idx on t_user (group_id) 

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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。 

<??> <!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> 

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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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