Java 为什么Hibernate为@manytone关联的隐式连接生成交叉连接?
鲍尔和金在他们的书中说: 隐式联接总是沿着多对一或一对一关联进行,而不是通过集合值关联 [第646页,第14章] 但当我在代码中这样做时,它生成的是交叉连接,而不是内部连接 映射来自Java 为什么Hibernate为@manytone关联的隐式连接生成交叉连接?,java,sql,database,hibernate,cross-join,Java,Sql,Database,Hibernate,Cross Join,鲍尔和金在他们的书中说: 隐式联接总是沿着多对一或一对一关联进行,而不是通过集合值关联 [第646页,第14章] 但当我在代码中这样做时,它生成的是交叉连接,而不是内部连接 映射来自Member2(多对一)->CLub 但是Club2没有关于成员的信息,并且Member2具有Club2的外键 我的问题是 // Implicit: Find all UK club member who is female Transaction t1 = HibernateUtil.begin(); Query
Member2
(多对一)->CLub
但是Club2
没有关于成员的信息,并且Member2
具有Club2的外键
我的问题是
// Implicit: Find all UK club member who is female
Transaction t1 = HibernateUtil.begin();
Query query =
HibernateUtil.getSession().createQuery("From Member2 m where m.club2.country = 'UK' ");
List<Member2> memList = query.list();
for (Member2 m : memList)
System.out.println(m);
HibernateUtil.end(t1);
Club2.java
package com.lilu.de.onetomany.uni.other;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.SequenceGenerator;
import javax.persistence.Table;
@Entity
@Table(name = "TBL_CLUB2")
public class Club2 {
@GeneratedValue(generator = "pkey_Club2", strategy = GenerationType.SEQUENCE)
@SequenceGenerator(name = "pkey_Club2", initialValue = 1000, allocationSize = 10,
sequenceName = "seq_pkey_Club2")
@Id
private int club_id;
private String club_name;
private String country;
// private Set Member2 = new HashSet();
public Club2() {
super();
}
public Club2(String cname, String ccountry) {
this.club_name = cname;
this.country = ccountry;
}
@Override
public String toString() {
String temp = club_name + " " + country + " ";
// Iterator<Member2> iter = Member2.iterator();
// String mems = null;
// while (iter.hasNext()) {
// mems += iter.next();
// }
// temp += "\n" + mems;
return temp;
}
public int getClub_id() {
return club_id;
}
public void setClub_id(int club_id) {
this.club_id = club_id;
}
public String getClub_name() {
return club_name;
}
public void setClub_name(String club_name) {
this.club_name = club_name;
}
public String getCountry() {
return country;
}
public void setCountry(String country) {
this.country = country;
}
/*
* public Set<Member2> getMember2() { return Member2; }
*
* public void setMember2(Set<Member2> Member2) { this.Member2 = Member2; }
*/
}
hibernate.cfg.xml
<?xml version='1.0' encoding='utf-8'?>
<!--
~ Hibernate, Relational Persistence for Idiomatic Java
~
~ Copyright (c) 2010, Red Hat Inc. or third-party contributors as
~ indicated by the @author tags or express copyright attribution
~ statements applied by the authors. All third-party contributions are
~ distributed under license by Red Hat Inc.
~
~ This copyrighted material is made available to anyone wishing to use, modify,
~ copy, or redistribute it subject to the terms and conditions of the GNU
~ Lesser General Public License, as published by the Free Software Foundation.
~
~ This program is distributed in the hope that it will be useful,
~ but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
~ or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License
~ for more details.
~
~ You should have received a copy of the GNU Lesser General Public License
~ along with this distribution; if not, write to:
~ Free Software Foundation, Inc.
~ 51 Franklin Street, Fifth Floor
~ Boston, MA 02110-1301 USA
-->
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<!-- Database connection settings -->
<property name="connection.driver_class">org.postgresql.Driver</property>
<property name="connection.url">jdbc:postgresql://localhost:5432/hibernatedb1</property>
<property name="connection.username">postgres</property>
<property name="connection.password">ani155</property>
<!-- JDBC connection pool (use the built-in) -->
<property name="connection.pool_size">1</property>
<!-- SQL dialect -->
<property name="dialect">org.hibernate.dialect.PostgreSQLDialect</property>
<!-- Disable the second-level cache -->
<property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>
<!-- Echo all executed SQL to stdout -->
<property name="show_sql">true</property>
<property name="format_sql">true</property>
<!-- Drop and re-create the database schema on startup -->
<property name="hbm2ddl.auto">update</property>
<!-- Names the annotated entity class -->
<mapping class="com.lilu.de.onetomany.uni.other.Club2"/>
<mapping class="com.lilu.de.onetomany.uni.other.Member2"/>
<!-- <mapping class="com.apal.mapping.onetoone.User"/> -->
</session-factory>
</hibernate-configuration>
org.postgresql.Driver
jdbc:postgresql://localhost:5432/hibernatedb1
博士后
ani155
1.
org.hibernate.dialogue.PostgreSqlDialogue
org.hibernate.cache.internal.NoCacheProvider
真的
真的
更新
Test.java
package com.lilu.de.onetomany.uni.other;
import java.util.List;
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
public class Test {
public static void main(String[] args) {
Test test = new Test();
// test.setup();
test.SelectQuery1Implicit();
// test.SelectQuery2ExplicitFromClause();
// test.SelectQuery3JoinFetch();
// test.SelectQuery4ThetaJoin();
}
private void SelectQuery1Implicit() {
// Implicit: Find all UK club member who is female
Transaction t1 = HibernateUtil.begin();
Query query =
HibernateUtil.getSession().createQuery("From Member2 m where m.club2.country = 'UK' ");
List<Member2> memList = query.list();
for (Member2 m : memList)
System.out.println(m);
HibernateUtil.end(t1);
}
private void setup() {
Transaction t1 = HibernateUtil.begin();
Club2 c1 = new Club2("MulaRougne", "UK");
Member2 m1 = new Member2("aaa", 25, 'm');
Member2 m2 = new Member2("bbb", 28, 'f');
m1.setClub2(c1);
m2.setClub2(c1);
HibernateUtil.getSession().save(c1);
HibernateUtil.getSession().save(m1);
HibernateUtil.getSession().save(m2);
Club2 c2 = new Club2("Queen's Club", "UK");
Club2 c3 = new Club2("Disney", "USA");
Member2 m3 = new Member2("ccc", 32, 'm');
Member2 m4 = new Member2("ddd", 23, 'm');
m3.setClub2(c3);
m4.setClub2(c3);
HibernateUtil.getSession().save(m3);
HibernateUtil.getSession().save(m4);
HibernateUtil.getSession().save(c2);
HibernateUtil.getSession().save(c3);
/*
* Club2 c1 = new Club2("MulaRougne", "UK"); Club2 c2 = new Club2("Queen's Club", "UK"); Club2
* c3 = new Club2("Disney", "USA");
*
* Member2 m1 = new Member2("aaa", 25, 'm'); Member2 m2 = new Member2("bbb", 28, 'f'); Member2
* m3 = new Member2("ccc", 32, 'm'); Member2 m4 = new Member2("ddd", 23, 'm'); Member2 m5 = new
* Member2("ee", 30, 'f');
*
* c1.getMember2().add(m1); c1.getMember2().add(m2); c1.getMember2().add(m3);
*
* c2.getMember2().add(m4); c2.getMember2().add(m5);
*
* HibernateUtil.getSession().save(c2); HibernateUtil.getSession().save(m4);
* HibernateUtil.getSession().save(m5);
*
* HibernateUtil.getSession().save(c1); HibernateUtil.getSession().save(m1);
* HibernateUtil.getSession().save(m2); HibernateUtil.getSession().save(m3);
*/
HibernateUtil.end(t1);
}
private static class HibernateUtil {
private static SessionFactory factory;
private static Session session;
static {
factory = new Configuration().configure().buildSessionFactory();
}
public static Session getSession() {
return session;
}
public static Transaction begin() {
session = factory.openSession();
return session.beginTransaction();
}
public static void end(Transaction tran) {
tran.commit();
}
}
}
package com.lilu.de.onetomany.uni.other;
导入java.util.List;
导入org.hibernate.Query;
导入org.hibernate.Session;
导入org.hibernate.SessionFactory;
导入org.hibernate.Transaction;
导入org.hibernate.cfg.Configuration;
公开课考试{
公共静态void main(字符串[]args){
测试=新测试();
//test.setup();
test.SelectQuery1Implicit();
//test.SelectQuery2ExplicitFromClause();
//test.选择query3joinfetch();
//测试。选择query4thetajoin();
}
私有void selectquery1隐式(){
//隐式:查找所有英国俱乐部女性成员
事务t1=HibernateUtil.begin();
查询=
HibernateUtil.getSession().createQuery(“From Member2 m,其中m.club2.country='UK'”);
List memList=query.List();
对于(成员2 m:memList)
系统输出打印项次(m);
冬眠结束(t1);
}
私有无效设置(){
事务t1=HibernateUtil.begin();
俱乐部2 c1=新俱乐部2(“英国穆拉鲁涅”);
成员2 m1=新成员2(“aaa”,25,“m”);
成员2 m2=新成员2(“bbb”,28,“f”);
m1.setClub2(c1);
m2.集合2(c1);
HibernateUtil.getSession().save(c1);
HibernateUtil.getSession().save(m1);
HibernateUtil.getSession().save(m2);
Club2 c2=新俱乐部2(“女王俱乐部”、“英国”);
Club2 c3=新的Club2(“迪士尼”,“美国”);
成员2 m3=新成员2(“ccc”,32,“m”);
成员2 m4=新成员2(“ddd”,23,“m”);
m3.setClub2(c3);
m4.集合2(c3);
HibernateUtil.getSession().save(m3);
HibernateUtil.getSession().save(m4);
HibernateUtil.getSession().save(c2);
HibernateUtil.getSession().save(c3);
/*
*俱乐部2 c1=新俱乐部2(“英国穆拉鲁涅”);俱乐部2 c2=新俱乐部2(“英国女王俱乐部”);俱乐部2
*c3=新俱乐部2(“迪士尼”、“美国”);
*
*成员2 m1=新成员2(“aaa”,25,“m”);成员2 m2=新成员2(“bbb”,28,“f”);成员2
*m3=新成员2(“ccc”,32,“m”);成员2 m4=新成员2(“ddd”,23,“m”);成员2 m5=新成员
*成员2(“ee”,30,“f”);
*
*c1.getMember2().add(m1);c1.getMember2().add(m2);c1.getMember2().add(m3);
*
*c2.getMember2().add(m4);c2.getMember2().add(m5);
*
*HibernateUtil.getSession().save(c2);HibernateUtil.getSession().save(m4);
*HibernateUtil.getSession().save(m5);
*
*HibernateUtil.getSession().save(c1);HibernateUtil.getSession().save(m1);
*HibernateUtil.getSession().save(m2);HibernateUtil.getSession().save(m3);
*/
冬眠结束(t1);
}
私有静态类HibernateUtil{
私营静电厂;
非公开静态会议;
静止的{
工厂=新配置().configure().buildSessionFactory();
}
公共静态会话getSession(){
返回会议;
}
公共静态事务开始(){
session=factory.openSession();
return session.beginTransaction();
}
公共静态无效结束(事务传输){
trans.commit();
}
}
}
大多数数据库引擎都会使用WHERE子句优化交叉连接,但我更喜欢使用显式连接
交叉连接由连接生成
:
where m.club2.country = 'UK'
为了避免第二次俱乐部查询,您可以按如下方式编写查询:
Query query = session.createQuery("""
select m
from Member2 m
join fetch m.club2 c
where
c.country = :country
""", Member2.class)
.setParameter("country", "UK");
使用绑定参数而不是硬编码参数时,此查询将删除和辅助选择。已尝试将HQL作为:
从Member2作为m内部连接m.club2作为c,其中c.country='UK'
有什么问题?where子句具有where member2x0_u0.club_id=club2x1_ux1.club_id
,因此Hibernate正确地从成员的俱乐部检索。一切都很好。是的,Hib正在正确地检索数据,但它正在进行交叉连接,而不是内部连接。我们知道交叉连接是一种速度慢且占用内存的交叉积。不应该是做一个内部连接??这是我关心的问题。。谢谢你的回复。@我知道我们可以做一个ecplicit内部连接From子句。根据这本书,hib应该为隐式连接生成一个内部连接。这里没有发生什么让我惊讶,想知道我错过了什么?嘿,谢谢你的回答。我知道那件事。但你澄清了hib无法生成内部连接,这是令人惊讶的。我会再试一次。看到你的博客,它是一块宝石。谢谢你的欣赏。我希望你读得愉快。当然。我要把它全部印出来。所有的文章很快就会被编成一本书。请继续收看@EmmanuelOgoma如果你有一个新问题,把它当作一个实际问题来问
where m.club2.country = 'UK'
Query query = session.createQuery("""
select m
from Member2 m
join fetch m.club2 c
where
c.country = :country
""", Member2.class)
.setParameter("country", "UK");