MySQL的索引失效原因 1、违反最左前缀法则 如果索引有多列,要遵守最左前缀法则 即查询从索引的最左前列开始并且不跳过索引中的列 explain select * from user where age = 20 and phone = '18730658760' and pos = 'cxy'; 2、在索引列上做任何操作 如计算、函数、(自动or手动)类型转换等操作,会导致索引失效从而全表扫描 explain select * from user where left(name,5) = 'zhangsan' and age = 20 and phone = '187306587