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Update 数据库索引.md

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Huimin Zhao 2021-03-14 03:38:45 +08:00 committed by GitHub
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@ -179,13 +179,6 @@ ALTER TABLE table ADD INDEX index_name (num,name,age)
那么当查询的条件有为:num / (num AND name) / (num AND name AND age)时，索引才生效。所以在创建联合索引时，尽量把查询最频繁的那个字段作为最左(第一个)字段。查询的时候也尽量以这个字段为第一条件。
> 但可能由于版本原因(我的 mysql 版本为 8.0.x),我创建的联合索引，相当于在联合索引的每个字段上都创建了相同的索引:
![联合索引(多列索引)](<../../media/pictures/database/联合索引(多列索引).png>)
无论是否符合最左前缀原则，每个字段的索引都生效:
![联合索引生效](../../media/pictures/database/联合索引之查询条件生效.png)
## 索引创建注意点