传智播客Hibernate视频教程学习笔记53

iterate查询与N+1次查询的问题

test.java

import hibernate.HibernateUtil;

import java.util.Date;
import java.util.Iterator;

import org.hibernate.Query;
import org.hibernate.Session;

import domain.Users;

public class test {

public static void main(String[] args) {
Users user = new Users();
user.setBirthday(new Date());
user.setName("pf1");
HibernateUtil.add(user);
user.setName("pf2");
HibernateUtil.add(user);
user.setName("pf3");
HibernateUtil.add(user);

iterator();

}

static void iterator(){
Session s = HibernateUtil.getSession();
Query query = s.createQuery("from Users");
Iterator<Users> it = query.iterate();
while(it.hasNext()){
System.out.println(it.next().getName());
}
}
}

执行结果：

Hibernate: insert into Users (ver, name, birthday) values (?, ?, ?)
Hibernate: insert into Users (ver, name, birthday) values (?, ?, ?)
Hibernate: insert into Users (ver, name, birthday) values (?, ?, ?)
Hibernate: select users0_.id as col_0_0_ from Users users0_
Hibernate: select users0_.id as id0_0_, users0_.ver as ver0_0_, users0_.name as name0_0_, users0_.birthday as birthday0_0_ from Users users0_ where users0_.id=?
pf1
Hibernate: select users0_.id as id0_0_, users0_.ver as ver0_0_, users0_.name as name0_0_, users0_.birthday as birthday0_0_ from Users users0_ where users0_.id=?
pf2
Hibernate: select users0_.id as id0_0_, users0_.ver as ver0_0_, users0_.name as name0_0_, users0_.birthday as birthday0_0_ from Users users0_ where users0_.id=?
pf3

可以发现红色那条语句，iterator查询是先查出记录的id号，在需要的时候再去查询，查询时候先从缓存里查找，再从数据库中读取。

由于我的Users是native方式生成主键，所以不会保存在缓存里，因为没访问数据库前不知道数据库生成的id号。

所有的select语句加起来是4条，包括查id的语句，所以一共是N+1次查询。所以缓存没命中的情况下查询效率极低。

同样懒加载的时候也会有N+1次查询的情况，第一次查询出所有当前对象，N次查询查询关联对象。