You can see the indexes of a table by viewing its definition. You will notice that there is an index on first_name and last_name. If you filter the rows by specifying first_name or by both (first_name and last_name), MySQL can use the index to speed up the query. However, if you specify only last_name, the index cannot be used; this is because the optimizer can only use any of the leftmost prefixes of the index. Refer to https://dev.mysql.com/doc/refman/8.0/en/multiple-column-indexes.html for more detailed examples:
mysql> ALTER TABLE employees ADD INDEX name(first_name, last_name);Query OK, 0 rows affected (2.23 sec)Records: 0 Duplicates: 0 Warnings: 0mysql> SHOW CREATE TABLE employees\G*************************** 1. row ...
