Order by text field with an index - using filesort - mysql

Question

I'm trying to order by and use an index but it seems to still be using filesort
Table
CREATE TABLE `teachers` (
`id` int(10) unsigned NOT NULL,
`name` VARCHAR(10) NOT NULL,
`lastName` tinytext NOT NULL,
KEY `lastName` (lastName(10))
)
Query
mysql> EXPLAIN SELECT name,lastName FROM teachers ORDER BY lastName DESC;
+----+-------------+----------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+------+---------------+------+---------+------+------+----------------+
| 1 | SIMPLE | teachers | ALL | NULL | NULL | NULL | NULL | 1546 | Using filesort |
+----+-------------+----------+------+---------------+------+---------+------+------+----------------+

Answer 1

You need to create a covering index last name, name
CREATE TABLE `teachers` (
`id` int(10) unsigned NOT NULL,
`name` varchar(10) NOT NULL,
`lastName` varchar(255) NOT NULL,
PRIMARY KEY (`id`),
KEY `last_first` (`lastName`,`name`)
)
Now that query
EXPLAIN SELECT `name`, lastName FROM teachers ORDER BY lastName DESC;
Produces the following result
+----+-------------+----------+-------+---------------+------------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+-------+---------------+------------+---------+------+------+-------------+
| 1 | SIMPLE | teachers | index | NULL | last_first | 269 | NULL | 1 | Using index |
+----+-------------+----------+-------+---------------+------------+---------+------+------+-------------+

Answer 2

Your problem is in key definition: KEY lastName (lastName(10))
Especially that you have defined a KEY length here
You index only a prefix of a column named in the ORDER BY clause. In this case, the index cannot be used to fully resolve the sort order. For example, if you have a CHAR(20) column, but index only the first 10 bytes, the index cannot distinguish values past the 10th byte and a filesort will be needed.
Proof link
You can write KEY lastName (lastName) and it will work as you need.