Question

(edited) For more details about the app it self, please, also see:
Simple but heavy application consuming a lot of resources. How to Optimize?
(The adopted solution was use both joins and fulltext search)

I have the following query running up to roughly 500.000 rows in 25 seconds. If I remove the ORDER, it takes 0.5 seconds.

Fisrt test

Keeping the ORDER and removing all t. and tu. columns, the query takes 7 seconds.

Second test

If I add or remove an INDEX to the i.created_at field the response time remain the same.

QUERY:

**EDITED: I’VE NOTICED THAT BOTH GROUP BY AND ORDER BY SLOW DOWN THE QUERY (I’ve also achieve a little gain in the query changing the joins. The gain was to 10secs, but at all, the problem remains). With the modification, the EXPLAIN have stopped to return filesort, but stills returning “using temporary” **

SELECT SQL_NO_CACHE
        DISTINCT `i`.`id`, 
        `i`.`entity`, 
        `i`.`created_at`, 
        `i`.`collected_at`, 

        `t`.`status_id` AS  `twt_status_id`, 
        `t`.`user_id` AS `twt_user_id`, 
        `t`.`content` AS `twt_content`, 
        `tu`.`id` AS `twtu_id`, 
        `tu`.`screen_name` AS `twtu_screen_name`, 
        `tu`.`profile_image` AS `twtu_profile_image`


        FROM `mtrt_items` AS `i`

        LEFT JOIN `mtrt_users` AS `u` ON i.user_id =u.id

        LEFT JOIN `twt_tweets_content` AS `t` ON t.id =i.id
        LEFT JOIN `twt_users` AS `tu` ON u.id = tu.id

        INNER JOIN `mtrt_items_searches` AS `r` ON i.id =r.item_id
        INNER JOIN `mtrt_searches` AS `s` ON s.id =r.search_id
        INNER JOIN `mtrt_searches_groups` AS `sg` ON sg.search_id =s.id
        INNER JOIN `mtrt_search_groups` AS `g` ON sg.group_id =g.id
        INNER JOIN `account_clients` AS `c` ON g.client_id =c.id                

    ORDER BY `i`.`created_at` DESC 
    LIMIT 100 OFFSET 0

Here is the EXPLAIN (EDITED):

+----+-------------+-------+--------+--------------------+-----------+---------+------------------------+------+------------------------------+
| id | select_type | table | type   | possible_keys      | key       | key_len | ref                    | rows | Extra                        |
+----+-------------+-------+--------+--------------------+-----------+---------+------------------------+------+------------------------------+
|  1 | SIMPLE      | c     | index  | PRIMARY            | PRIMARY   | 4       | NULL                   |    1 | Using index; Using temporary |
|  1 | SIMPLE      | g     | ref    | PRIMARY,client_id  | client_id | 4       | clubr_new.c.id         |    3 | Using index                  |
|  1 | SIMPLE      | sg    | ref    | group_id,search_id | group_id  | 4       | clubr_new.g.id         |    1 | Using index                  |
|  1 | SIMPLE      | s     | eq_ref | PRIMARY            | PRIMARY   | 4       | clubr_new.sg.search_id |    1 | Using index                  |
|  1 | SIMPLE      | r     | ref    | search_id,item_id  | search_id | 4       | clubr_new.s.id         | 4359 | Using where                  |
|  1 | SIMPLE      | i     | eq_ref | PRIMARY            | PRIMARY   | 8       | clubr_new.r.item_id    |    1 |                              |
|  1 | SIMPLE      | u     | eq_ref | PRIMARY            | PRIMARY   | 8       | clubr_new.i.user_id    |    1 | Using index                  |
|  1 | SIMPLE      | t     | eq_ref | PRIMARY            | PRIMARY   | 4       | clubr_new.i.id         |    1 |                              |
|  1 | SIMPLE      | tu    | eq_ref | PRIMARY            | PRIMARY   | 8       | clubr_new.u.id         |    1 |                              |
+----+-------------+-------+--------+--------------------+-----------+---------+------------------------+------+------------------------------+

Here is the mtrt_items table:

+--------------+-------------------------------------------------------+------+-----+---------+----------------+
| Field        | Type                                                  | Null | Key | Default | Extra          |
+--------------+-------------------------------------------------------+------+-----+---------+----------------+
| id           | bigint(20)                                            | NO   | PRI | NULL    | auto_increment |
| entity       | enum('twitter','facebook','youtube','flickr','orkut') | NO   | MUL | NULL    |                |
| user_id      | bigint(20)                                            | NO   | MUL | NULL    |                |
| created_at   | datetime                                              | NO   | MUL | NULL    |                |
| collected_at | datetime                                              | NO   |     | NULL    |                |
+--------------+-------------------------------------------------------+------+-----+---------+----------------+

 CREATE TABLE `mtrt_items` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `entity` enum('twitter','facebook','youtube','flickr','orkut') COLLATE utf8_unicode_ci NOT NULL,
  `user_id` bigint(20) NOT NULL,
  `created_at` datetime NOT NULL,
  `collected_at` datetime NOT NULL,
  PRIMARY KEY (`id`),
  KEY `mtrt_user_id` (`user_id`),
  KEY `entity` (`entity`),
  KEY `created_at` (`created_at`),
  CONSTRAINT `mtrt_items_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `mtrt_users` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=309650 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

The twt_tweets_content is MyISAM and is also used for fulltext searches:

+-----------+--------------+------+-----+---------+-------+
| Field     | Type         | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+-------+
| id        | int(11)      | NO   | PRI | NULL    |       |
| user_id   | int(11)      | NO   | MUL | NULL    |       |
| status_id | varchar(100) | NO   | MUL | NULL    |       |
| content   | varchar(200) | NO   | MUL | NULL    |       |
+-----------+--------------+------+-----+---------+-------+

Answer 1

Instead of placing the Order By into the main query, wrap it, like so:

SELECT * FROM (   
  ... your query
) ORDER BY `created at`

Take a look at the query plan. You will find that in your case, the sort is performed on your table mtrt_items before the outer join is performed. In the rewrite I’ve partially provided, the sort is applied after the outer joins, and is applied on a much smaller set.

UPDATE

Assuming that the LIMIT is being applied to a large set (500,000?), it looks like you can perform the top before doing any of the joins.

SELECT * from (
    SELECT 
    `id`, ... `created_at`, ...
    ORDER BY `i`.`created_at` DESC 
    LIMIT 100 OFFSET 0) as i

    LEFT JOIN `mtrt_users` AS `u` ON i.user_id =u.id

    LEFT JOIN `twt_tweets_content` AS `t` ON t.id =i.id
    LEFT JOIN `twt_users` AS `tu` ON t.user_id = tu.id

    INNER JOIN `mtrt_items_searches` AS `r` ON i.id =r.item_id
    INNER JOIN `mtrt_searches` AS `s` ON s.id =r.search_id
    INNER JOIN `mtrt_searches_groups` AS `sg` ON sg.search_id =s.id
    INNER JOIN `mtrt_search_groups` AS `g` ON sg.group_id =g.id
    INNER JOIN `account_clients` AS `c` ON g.client_id =c.id                

GROUP BY i.id