Condition of Index invalidation

Indexes do not always take effect, such as the following, which will invalidate the index:

1. If there is an or in the condition, it will not be used even if there is a conditional index (which is why the use of or is minimized)

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

2. For multi-column indexes, not the first part of use, the index is not used

3.like queries are preceded by%

4. If the column type is a string, be sure to use quotation marks in the condition to reference the data, otherwise you will not use the index

5. If MySQL estimates that using a full table scan is faster than using an index, the index is not used

Also, view the usage of the index
Show status like ' handler_read% ';
We can note:
Handler_read_key: The higher the value the better, the higher the number of times to use the index query
Handler_read_rnd_next: The higher the value, the less efficient the query