mysql 中!= 到底走不走索引？

       很多人疑惑!= 到底走不走索引， 这里可以肯定的说该操作是可以走索引的，但实际情况中都为啥都不走索引呢？ 首先我们要知道走索引与数据量和数据趋势（cardinality）有很大的关系，如果表本身就上百条记录，那走索引和表扫描区别不大，甚至在存在书签跳转情况下还不如表扫描更有效率，这个时候可能是不走索引的，另外 !=操作后获取的结果集在总结果集中占据的比例也是关键因素，如果返回的结果集过大（大于20%），那么可能也不会走索引，而是选择更有效率的表扫描了。

我们来看看个例子：

CREATE TABLE `b` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `age` bigint(20) DEFAULT 50,
  `name` varchar(50) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `name` (`name`)
) ENGINE=InnoDB

insert into b (name) values('a'),('b'),('c'),('d'),('e'),('f'),('g'),('h'),('i'); 
--多次执行
insert into b (name) 
select 'k' from b ;

select name, count(*) 
from  b 
group by name 

explain
select * From b where name !='k';

explain
select * From b where name !='a';

很明显 我们从统计中看到 K的数值很大，所以我们!='k'的数据集才8条，在总数据中占比很小，可以很好的走索引，而如果将k替换成 a，则不会走索引，变成了全表扫描

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