二十三 iterate查询与N+1次查询的问题
test.javaview plaincopy to clipboardprint?01.import hibernate.HibernateUtil; 02. 03.import java.util.Date; 04.import java.util.Iterator; 05. 06.import org.hibernate.Query; 07.import org.hibernate.Session; 08. 09.import domain.Users; 10. 11. 12. 13.public class test { 14. 15. 16. 17. public static void main(String[] args) { 18. Users user = new Users(); 19. user.setBirthday(new Date()); 20. user.setName("pf1"); 21. HibernateUtil.add(user); 22. user.setName("pf2"); 23. HibernateUtil.add(user); 24. user.setName("pf3"); 25. HibernateUtil.add(user); 26. 27. iterator(); 28. 29. 30. } 31. 32. static void iterator(){ 33. Session s = HibernateUtil.getSession(); 34. Query query = s.createQuery("from Users"); 35. Iterator<Users> it = query.iterate(); 36. while(it.hasNext()){ 37. System.out.println(it.next().getName()); 38. } 39. } 40.} 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=?pf1Hibernate: 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=?pf2Hibernate: 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。懒加载也会有N+1次查询当有个部门它下面有十个员工 我们去查这个部门，当要查询这个部门下的员工时，就会出现11条select语句，一条是这个部门的查询 另外十条是根据部门id查员工的select语句如果我查询员工，通过员工去查部门，就不会有N+1次查询，因为这在查员工的时候就把部门信息给查出来了 这种情况在一对一的关系中特别明显总结 N+1次查询和懒加载 1.用Query.iterator可能会有N+1次查询。 2.懒加载时获取关联对象。 3.如果打开对查询的缓存即使用list也可能有N+1次查询。完毕 end!?