1. There must be an all-or index;
2. The composite index unused fields in the left column;
3.like beginning%;
4. required type conversion;
5.where listed in the index calculation;
6.where as a function of the index of columns ;
7. when a full table scan faster if mysql feel (less data);
Seven kinds of index cases of failure
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Origin www.cnblogs.com/zheng123/p/11319507.html
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