MySQL第11天：MySQL索引优化分析之性能分析

迈不过友情╰ 2022-10-22 07:52 189阅读 0赞

###                     MySQL索引优化分析之性能分析 ###

### 一、MySQL Query Optimizer   二、MySQL常见瓶颈   三、Explain（执行计划） ###

1、什么是执行计划？  
         2、执行计划能干什么？  
         3、执行计划怎么使用？  
         4、执行计划中各字段解释

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# 一、MySQL Query Optimizer #

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70][]

# 二、MySQL常见瓶颈  #

![20210405165341850.png][]

# 三、Explain（执行计划） #

### 1、什么是执行计划？ ###

使用Explain关键字可以模拟优化器执行SQL查询语句，从而知道MySQL如何处理你的SQL语句。分析你的查询语句或是表结构的性能瓶颈。

### 2、执行计划能干什么？ ###

（1）表的读取顺序
    （2）数据读取操作的操作类型
    （3）哪些索引可以使用
    （4）哪些索引被实际使用
    （5）表之间的引用
    （6）每张表有多少行被优化器查询
    
    PS：
    
    （1）select_type：数据读取操作的操作类型
    
    （2）possible_keys：哪些索引可以使用
    
    （3）key：哪些索引被实际使用
    
    （4）rows：每张表有多少行被优化器查询（rows越少越好）

### 3、执行计划怎么使用？ ###

**     1）Explain+SQL语句  
      2）执行计划包含的信息**

![20210405171055178.png][]

### 4、执行计划中各字段解释 ###

### **字段1：id：** ###

1）select查询的序列号，包含一组数字，表示查询中执行select子句或操作表的顺序。  
2）三种情况：  
第一种情况id相同：执行顺序由上而下；

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 1][]  
第二种情况id不同：如果是子查询，id的序号会递增，id值越大优先级越高，越先被执行

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 2][]  
第三种情况id相同和不同，同时存在

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 3][]

### **字段2：**select\_type ###

### ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 4][] ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 5][]

### **字段3：**table ###

显示这一行的数据是关于哪张表的

### **字段4：**type ###

### ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 6][]   ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 7][] ###

![20210405181610243.png][]

### ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 8][] ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 9][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 10][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 11][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 12][]

### ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 13][] ###

### ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 14][] ###

### **字段5：**possible\_keys ###

理论上用到的索引。

### ![20210405182622651.png][]   **字段6：**key ###

实际用到的索引。

### ![20210405182717521.png][]   ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 15][] ###

### **字段7：**key\_len ###

![2021040518330446.png][]

### ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 16][]   **字段8：**ref ###

### ![20210405183832811.png][]   ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 17][] ###

### **字段9：**rows ###

### ![20210405184323350.png][]   ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 18][] ###

### **字段10：**extra ###

extra：包含不适合在其他列中显示但十分重要的额外信息。常见的有：
    （1）using filesort
    （2）using temporary
    （3）using index
    （4）using where
    （5）using join buffer
    （6）impossible where
    （7）select tables optimized away
    （8）distinct

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 19][]

![20210405202115819.png][]

![20210405202605406.png][]

![20210405203041601.png][]

PS：Using temporary：使用了临时表保存中间结果，MySQL在对查询结果排序时使用临时表，常见于排序order by和分组查询group by。

### 1、Using  filesort ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 20][]

### 2、Using temporary ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 21][]

PS：group by尽量根据索引的个数和顺序来，否则会产生文件列排序和临时表（using filesort、using temorary），影响查询性能。

### 3、Using  index ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 22][]

### ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 23][] ###

### 4、using join buffer ###

使用了链接缓存

### 5、impossible where ###

where子句的值总是false，不能用来获取任何元组

### 6、impossible where ###

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaWJvMTIzMDEyMw_size_16_color_FFFFFF_t_70 24][]

### 7、select tables optimized away ###

### ![20210405202605406.png][] ###

### 8、distinct ###

![20210405203041601.png][]

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