MySql 索引优化和避免索引失效

古城微笑少年丶 2023-02-15 12:44 45阅读 0赞

建表和索引  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70]

## 索引优化法则 ##

![1、全值匹配我最爱][1]  
**全值匹配案例：**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 1]

**最佳左前缀法则：**  
![在这里插入图片描述][20200605150542207.png]

如果中间断了，那么只部分使用了索引，只有前面的的字段使用了索引

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 2]

**不在索引列上做任何操作（计算，函数，…）**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 3]

**索引中范围条件右边的列全失效**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 4]

**尽量使用覆盖索引**

覆盖索引，直接去索引中取数据，不去表中取数据，大大提高效率  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 5]

**mysql 在使用不等于（!= 或者 <>）的时候无法使用索引导致全表扫描**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 6]

**is null , not null 也无法使用索引**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 7]

**like以通配符开头（‘%abc…’） mysql索引失效会变成全表扫描**

案例如下： %只有写在最右边的索引才会生效  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 8]

解决like '%字符串%'时索引不被使用的方法？（使用覆盖索引解决）

> 使用覆盖索引，查询的字段尽量是索引的字段  
> ![在这里插入图片描述][2020060515572747.png]  
> 未使用覆盖索引会失效,查询的字段没有索引  
> email 没有使用索引![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 9]

**字符串不加单引号索引失效**  
![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 10][]

**少用or,用来连接时会导致索引失效**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 11]

## 总结 ##

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 12]

## 案例 ##

ordey by 排序 也要是 最佳左前缀法则，不然排序会失效  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3_size_16_color_FFFFFF_t_70 13]  
![在这里插入图片描述][20200605171008438.png]

## 索引优化建议 ##

![在这里插入图片描述][2020060517090442.png]

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[1]: https://img-blog.csdnimg.cn/20200605145611553.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Z5ajEzOTI1NDc1OTU3,size_16,color_FFFFFF,t_70
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[20200605171008438.png]: https://img-blog.csdnimg.cn/20200605171008438.png
[2020060517090442.png]: https://img-blog.csdnimg.cn/2020060517090442.png