MySQL learns the extra in-explain

Overriding an index is exactly the same number of fields that are created for the index and query

Using Filesort:mysql uses an external index instead of the in-table index order to access the sort operations that MySQL cannot do with the index, which is called a file index, and if you create an index but do not query in the order of the index, it will appear

Using temporary: Using temporary save intermediate results, MySQL uses temporal tables when sorting query results, and is common in sort order by and group by queries.

Using index: Overwrite index used

MySQL learns the extra in-explain