Question

I have a problem where my query is Using filesort, even though the field is indexed.

This is my query:

EXPLAIN
SELECT u.name, u.surname, u.id
FROM rw_users u
ORDER BY u.created DESC
LIMIT 4

This is the output:

id  select_type  table  type  possible_keys  key  key_len  ref  rows  Extra           
1   SIMPLE       u      ALL                                     9     Using filesort 

This is create table:

CREATE TABLE `rw_users` (
  `id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
  `created` int(11) unsigned NOT NULL,
  `email` varchar(255) COLLATE utf8_swedish_ci NOT NULL,
  `password` char(64) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL,
  `name` varchar(100) COLLATE utf8_swedish_ci NOT NULL,
  `surname` varchar(100) COLLATE utf8_swedish_ci NOT NULL,
  `premium` tinyint(1) unsigned NOT NULL DEFAULT '1' COMMENT 'level or premium membership (0 = standard)',
  PRIMARY KEY (`id`),
  UNIQUE KEY `email` (`email`),
  KEY `password` (`password`),
  KEY `created` (`created`),
  KEY `premuim` (`premium`),
  KEY `name_surname` (`name`,`surname`),
  CONSTRAINT `FK_rw_users` FOREIGN KEY (`premium`) REFERENCES `rw_premium` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=14 DEFAULT CHARSET=utf8 COLLATE=utf8_swedish_ci

What do I need to do to get rid of Using filesort? I tested on database (exactly the same table) with 118 rows and it still gives me Using filesort, so it can’t use it because its faster than using index.

Answer 1

For the best performance for select queries add covering index. See here:

ALTER TABLE rw_users ADD KEY ix1 (created, name, surname, id);

for INNODB engine you don’t need to include id in index, check both indexes with EXPLAIN.

ALTER TABLE rw_users ADD KEY ix1 (created, name, surname);