Using index, using temporary, using filesort - how to fix this?
Well, the doc gives the exact reasons when "Using temporary" will appear:
Temporary tables can be created under conditions such as these:
If there is an ORDER BY clause and a different GROUP BY clause, or if the ORDER BY or GROUP BY contains columns from tables other than the first table in the join queue, a temporary table is created.
DISTINCT combined with ORDER BY may require a temporary table.
If you use the SQL_SMALL_RESULT option, MySQL uses an in-memory temporary table, unless the query also contains elements (described later) that require on-disk storage.
A quick scan shows that you suffer from #1.
And this blog from 2009 says that "using filesort" means that the sort can't be performed with an index. Since you're ordering by a computed field, that's going to be true, too.
So, that's what's "wrong".
Updated for MySQL 5.7 (src):
The server creates temporary tables under conditions such as these:
Evaluation of UNION statements, with some exceptions described later.
Evaluation of some views, such those that use the TEMPTABLE algorithm, UNION, or aggregation.
Evaluation of derived tables (subqueries in the FROM clause).
Tables created for subquery or semi-join materialization (see Section 8.2.2, “Optimizing Subqueries, Derived Tables, and View References”).
Evaluation of statements that contain an ORDER BY clause and a different GROUP BY clause, or for which the ORDER BY or GROUP BY contains columns from tables other than the first table in the join queue.
Evaluation of DISTINCT combined with ORDER BY may require a temporary table.
For queries that use the SQL_SMALL_RESULT modifier, MySQL uses an in-memory temporary table, unless the query also contains elements (described later) that require on-disk storage.
To evaluate INSERT ... SELECT statements that select from and insert into the same table, MySQL creates an internal temporary table to hold the rows from the SELECT, then inserts those rows into the target table. See Section 13.2.5.1, “INSERT ... SELECT Syntax”.
Evaluation of multiple-table UPDATE statements.
Evaluation of GROUP_CONCAT() or COUNT(DISTINCT) expressions.