What is the situation of index failure caused by mysql

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1. Calculate the usage function or operation of the index column in the query condition.

If an established index field performs a function operation while in use, the index cannot be used.

Because the B+ tree maintained by MySQL for the index is based on the raw data of the field, if you add a function during use, MySQL will not consider the field to be the original field, so it will certainly not be used.

SELECT * FROM student WHERE round (age) = 2

2. Start with% under the like condition.

Select * from student where name like'% bc%'

3. Leftmost matching principle.

The leftmost matching principle means that in a federated index, if the leftmost index in the federated index is used in your sql sentence, the sql sentence can be matched by the federated index when a range query is encountered (>