Is the MySQL index functioning properly?

The effectiveness of MySQL indexes depends on their usage and query optimization. Here are some common scenarios in which indexes are effective.

1. When using the indexed fields in a query, MySQL will utilize the index to speed up the query and improve query performance.
2. Index selectivity: Index selectivity refers to the proportion of unique values in an index column. The higher the selectivity, the better the index performance. For example, an index column with only two different values has low selectivity, so the index may not be effective.
3. Index cover query: When a query only needs to use the data in the indexed columns and does not need to access other data in the table, it is called an index cover query. In this scenario, MySQL does not need to access the data rows in the table, only the index, which can greatly improve query performance.
4. If functions or expressions are used in a query, MySQL may not be able to accelerate the query using indexes.
5. The accuracy of statistical information: MySQL uses statistical information to optimize query plans, if the statistics are inaccurate, it may result in indexes not being effective.
6. Amount of data in the table: for smaller tables, even without an index, query performance may not be significantly affected. However, for larger tables, the use of indexes is crucial in improving query performance.

In conclusion, to ensure the effectiveness of indexes, it is necessary to select appropriate indexes based on actual conditions, as well as optimize queries and maintain statistics.