Under what circumstances will the index fail? , situation index failure?

Under what circumstances will the index fail? , situation index failure?

The index does not take effect all the time. For example,the following situations will cause the index to fail:

　

Note: If you want to use or and want the index to take effect, you can only add an index to each column in the or condition

2. For multi-column indexes, if it is not the first part used, the index will not be used

　3.Like query starts with %

4. If the column type is a string, the data must be quoted in quotation marks in the condition, otherwise the index will not be used

　

5. If mysql estimates that using a full table scan is faster than using an index, then do not use the index

Also, view index usage

show status like ‘Handler_read%’;
You can note:
handler_read_key: The higher the value, the better. The higher the value, the higher the number of queries using the index. handler_read_rnd_next: The higher the value, the query is inefficient