How to check if the indexes created on a MySQL database are invalid?
To check if the indexes in a MySQL database are ineffective, you can use the following method:
1. Use the EXPLAIN statement: Add the EXPLAIN keyword before the query statement to see the indexes used by the MySQL optimizer when executing the query. If the indexes are ineffective, the optimizer may choose a full table scan instead of using the index. For example, for the following query statement:
EXPLAIN SELECT * FROM table_name WHERE column_name = 'value';
If an index is invalid, the type column in the EXPLAIN result may show as ALL, indicating a full table scan. Another option is to use the SHOW INDEX statement: With the SHOW INDEX statement, you can view index information in the table. If you notice that the Cardinality value of a certain index is very low or the data distribution in the table makes it so that the index cannot effectively filter data, it may be necessary to reconsider creating or adjusting the index. For example, for the following query:
SHOW INDEX FROM table_name;
You can view index information in the table, including the index name, column name, uniqueness, cardinality, etc. Using the slow query log: MySQL’s slow query log records queries that exceed a specified threshold execution time, and you can check if indexes are ineffective by looking at the slow query log. First, make sure the slow query log is enabled. You can enable the slow query log by adding the following content to the my.cnf configuration file.
slow_query_log = 1slow_query_log_file = /path/to/slow_query.log
long_query_time = 1
Then, you can use the following command to view the contents of the slow query log:
sudo tail -f /path/to/slow_query.log
If a query is found to have a long execution time, it may be necessary to check if the index used by that query is invalid. One option is to use MySQL’s built-in tools and commands, such as mysqldumpslow, mysqlindexcheck, and pt-duplicate-key-checker, to analyze query logs and index usage in order to determine if an index is invalid. In conclusion, to check if indexes in a MySQL database are invalid, you can use the EXPLAIN statement, SHOW INDEX statement, slow query logs, and MySQL’s built-in tools and commands for inspection.
