如何避免在 "!=" mysql 中使用 Filesort

Question

Please, help me! How to optimize a query like:

SELECT idu
FROM `user`
WHERE `username`!='manager'
  AND `username`!='user1@yahoo.com'
ORDER BY lastdate DESC

This is the explain:

explain SELECT idu FROM `user` WHERE `username`!='manager' AND `username`!='ser1@yahoo.com' order by lastdate DESC;
+----+-------------+-------+------+----------------------------+------+---------+------+--------+-----------------------------+
| id | select_type | table | type | possible_keys              | key  | key_len | ref  | rows   | Extra                       |
+----+-------------+-------+------+----------------------------+------+---------+------+--------+-----------------------------+
|  1 | SIMPLE      | user  | ALL  | username,username-lastdate | NULL | NULL    | NULL | 208478 | Using where; Using filesort | 
+----+-------------+-------+------+----------------------------+------+---------+------+--------+-----------------------------+
1 row in set (0.00 sec)    

To avoid file sorting in a big database.

Answer 1

Since this query is just scanning all rows, you need an index on lastdate to avoid MySQL from having to order the results manually (using filesort, which isn't always to disk/temp table).

For super read performance, add the following multi-column "covering" index:

user(lastdate, username, idu)

A "covering" index would allow MySQL to just scan the index instead of the actual table data.

If using InnoDB and any of the above columns are your primary key, you don't need it in the index.