ibatis N+1问题

我们先来看一个问题：一个User用户对应多张信用卡Card

类User:

package com.demo.beans;

import java.util.List;

public class User {
	private int id;
	private String name;
	private List<Card> cards;

        set get省略...
}

 类Card:

package com.demo.beans;

public class Card {
	private int id;
	private String cardName;
	private double balance;
	private int userId;

        set get省略...
}

对应数据库中的表结构：

表user:

CREATE TABLE `user` (
  `Id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`Id`)
) ENGINE=InnoDB AUTO_INCREMENT=1004 DEFAULT CHARSET=utf8;

 表card:

CREATE TABLE `card` (
  `Id` int(11) NOT NULL AUTO_INCREMENT,
  `cardName` varchar(255) DEFAULT NULL,
  `balance` double DEFAULT NULL,
  `userId` int(11) DEFAULT NULL,
  PRIMARY KEY (`Id`)
) ENGINE=InnoDB AUTO_INCREMENT=2006 DEFAULT CHARSET=utf8;

 

我们考虑这样一个需求：从数据库中获得所有用户的信息（包括他所持有的信用卡信息）。

对应的UserCards.xml:

<?xml version="1.0" encoding="UTF-8" ?>

<!DOCTYPE sqlMap      
    PUBLIC "-//ibatis.apache.org//DTD SQL Map 2.0//EN"      
    "http://ibatis.apache.org/dtd/sql-map-2.dtd">

<sqlMap namespace="UserCards">

	<typeAlias alias="Card" type="com.demo.beans.Card" />
	<typeAlias alias="User" type="com.demo.beans.User"/>
	
	<resultMap class="User" id="UserResult" >
		<result property="id" column="id"/>
		<result property="name" column="name"/>
		<result property="cards" select="UserCards.getCardsByUserId" column="id"/>
	</resultMap>
	
	<resultMap id="CardResult" class="Card">
		<result property="id" column="id" />
		<result property="cardName" column="cardName" />
		<result property="balance" column="balance" />
		<result property="userId" column="userId" />
	</resultMap>
	
	<select id="getUserById" resultMap="UserResult" parameterClass="int">
		select * from user where id = #value#
	</select>
	
	<select id="getCardsByUserId" resultMap="CardResult" parameterClass="int">
		select * from card where userId = #value#
	</select>
	
	<select id="getAllUsers" resultMap="UserResult">
		select * from user
	</select>
	
	<insert id="addCard" parameterClass="Card" >
		<selectKey resultClass="int" type="post"  keyProperty="id" >
			select LAST_INSERT_ID() as value
		</selectKey>
		insert into card values(#id#,#cardName#,#balance#,#userId#)
	</insert>
	
	<insert id="addUser" parameterClass="User" >
		<selectKey resultClass="int" type="post"  keyProperty="id" >
			select LAST_INSERT_ID() as value
		</selectKey>
		insert into user values(#id#,#name#)
	</insert>
	
</sqlMap>

 

注意上面xml关键部分：

<result property="cards" select="UserCards.getCardsByUserId" column="id"/>

每查一个user，根据他的id来getCardsByUserId查卡信息

 

我们的单元测试部分：

public void testGetAll(){
		BeanFactory factory = new ClassPathXmlApplicationContext("applicationContext.xml");
		UserDao userDao = (UserDao) factory.getBean("userDao");
		
		long start = System.currentTimeMillis();
		
		List<User> userList = userDao.getAllUser();
		for(User user : userList){
			System.out.println(user.getName());
			for(Card card : user.getCards()){
				System.out.println(card.getCardName()+"|"+card.getBalance());
			}
			System.out.println("------------------------");
		}
		
		long end = System.currentTimeMillis();
		System.out.println(end-start);
	}

 

在数据库中我们添加了1000个左右的用户和1000张卡信息，相互对应

控制台输出如下：

xiaoye994
2011-01-14 10:38:52,218 DEBUG - {conn-102994} Connection
2011-01-14 10:38:52,218 DEBUG - {conn-102994} Preparing Statement:    select * from card where userId = ?  
2011-01-14 10:38:52,218 DEBUG - {pstm-102995} Executing Statement:    select * from card where userId = ?  
2011-01-14 10:38:52,218 DEBUG - {pstm-102995} Parameters: [998]
2011-01-14 10:38:52,218 DEBUG - {pstm-102995} Types: [java.lang.Integer]
中信A00998|999.0
------------------------
xiaoye995
2011-01-14 10:38:52,218 DEBUG - {conn-102997} Connection
2011-01-14 10:38:52,218 DEBUG - {conn-102997} Preparing Statement:    select * from card where userId = ?  
2011-01-14 10:38:52,218 DEBUG - {pstm-102998} Executing Statement:    select * from card where userId = ?  
2011-01-14 10:38:52,218 DEBUG - {pstm-102998} Parameters: [999]
2011-01-14 10:38:52,218 DEBUG - {pstm-102998} Types: [java.lang.Integer]
中信A00999|1000.0
------------------------
2078

 

可以看到每次输出一个用户名如"xiaoye995"，然后发出一条select * from card where userId = ?语句。

这就是N+1问题：我们有995个用户 那么就会发出995条上面的语句 另外一条则是查询用户的SQL语句select * from user，

这里的N即为995。可以看到，如果要查询所有用户及其卡信息，这样的效率是很差的，总共花了大约2000多毫秒的时间。

 

解决办法之一，使用懒加载 就是在需要用户卡信息的时候才触发sql调用

<settings lazyLoadingEnabled="true" useStatementNamespaces="true"/>

解决办法之二，ibatis中已经解决了N+1问题，具体看下面：

修改UserCards.xml文件，关键部分如下：

<resultMap class="User" id="UserResult" groupBy="id">
		<result property="id" column="id" nullValue="0"/>
		<result property="name" column="name"/>
		<result property="cards" resultMap="UserCardsN1.CardResult" />
</resultMap>

<select id="getAllUsers" resultMap="UserResult">
		select * from user a left join card b on a.id = b.userId
</select>

 

区别主要体现在groupBy="id"和<result property="cards" resultMap="UserCardsN1.CardResult" />及select用了表连接

控制台输出信息：

2011-01-14 11:10:55,156 DEBUG - {conn-100000} Connection
2011-01-14 11:10:55,171 DEBUG - {conn-100000} Preparing Statement:    select * from user a left join card b on a.id = b.userId  
2011-01-14 11:10:55,703 DEBUG - {pstm-100001} Executing Statement:    select * from user a left join card b on a.id = b.userId  
2011-01-14 11:10:55,703 DEBUG - {pstm-100001} Parameters: []
2011-01-14 11:10:55,703 DEBUG - {pstm-100001} Types: []
xiaoye
建行卡|4353.0
招行卡|23112.0
中信A001|2.0
------------------------
king
工行卡|1000.0
中信A002|3.0
------------------------
..........
.........
------------------------
766

 可以看到只发出了一条sql语句 输出了所有的信息 大概用时766毫秒

 

结论：

1）懒加载适合不需要一次性取出用户信息及其关联信息的场景

 

2）N+1的解决方法是一次性取出的，适合数据量较少的情况