ibatis N+1問題

我們先來看一個問題：一個User用戶對應多張信用卡Card

類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...
}

類Card:

package com.demo.beans;

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

        set get省略...
}

 

對應數據庫中的表結構：

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;

 

我們考慮這樣一個需求：從數據庫中獲得所有用戶的信息（包括他所持有的信用卡信息）。

對應的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關鍵部分：

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

每查一個user，根據他的id來getCardsByUserId查卡信息

 

我們的單元測試部分：

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);
	}

 

在數據庫中我們添加了1000個左右的用戶和1000張卡信息，相互對應

控制檯輸出如下：

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

 

可以看到每次輸出一個用戶名如"xiaoye995"，然後發出一條select * from card where userId = ?語句。

這就是N+1問題：我們有995個用戶 那麼就會發出995條上面的語句 另外一條則是查詢用戶的SQL語句select * from user，

這裏的N即爲995。可以看到，如果要查詢所有用戶及其卡信息，這樣的效率是很差的，總共花了大約2000多毫秒的時間。

 

解決辦法之一，使用懶加載 就是在需要用戶卡信息的時候才觸發sql調用

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

 

解決辦法之二，ibatis中已經解決了N+1問題，具體看下面：

修改UserCards.xml文件，關鍵部分如下：

<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>

 

區別主要體現在groupBy="id"和<result property="cards" resultMap="UserCardsN1.CardResult" />及select用了表連接

控制檯輸出信息：

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

可以看到只發出了一條sql語句 輸出了所有的信息 大概用時766毫秒

 

結論：

1）懶加載適合不需要一次性取出用戶信息及其關聯信息的場景

 

2）N+1的解決方法是一次性取出的，適合數據量較少的情況

 

http://xiaoye4188.iteye.com/blog/871050