MySQL powolne kwerendy: INNER JOIN + BY przyczyny filesort

Question

Próbuję zoptymalizować zapytanie:

 
SELECT `posts`.* FROM `posts` INNER JOIN `posts_tags` 
    ON `posts`.id = `posts_tags`.post_id 
    WHERE (((`posts_tags`.tag_id = 1))) 
    ORDER BY posts.created_at DESC; 

Wielkość tabel jest 38k wierszy i 31K i MySQL używa „filesort” tak robi się dość powolny. Próbowałem użyć różnych indeksów, bez powodzenia.

 
CREATE TABLE `posts` (
    `id` int(11) NOT NULL auto_increment, 
    `created_at` datetime default NULL, 
    PRIMARY KEY (`id`), 
    KEY `index_posts_on_created_at` (`created_at`), 
    KEY `for_tags` (`trashed`,`published`,`clan_private`,`created_at`) 
) ENGINE=InnoDB AUTO_INCREMENT=44390 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci 

CREATE TABLE `posts_tags` (
    `id` int(11) NOT NULL auto_increment, 
    `post_id` int(11) default NULL, 
    `tag_id` int(11) default NULL, 
    `created_at` datetime default NULL, 
    `updated_at` datetime default NULL, 
    PRIMARY KEY (`id`), 
    KEY `index_posts_tags_on_post_id_and_tag_id` (`post_id`,`tag_id`) 
) ENGINE=InnoDB AUTO_INCREMENT=63175 DEFAULT CHARSET=utf8 

 
+----+-------------+------------+--------+--------------------------+--------------------------+---------+---------------------+-------+-----------------------------------------------------------+ 
| id | select_type | table  | type | possible_keys   | key      | key_len | ref     | rows | Extra              | 
+----+-------------+------------+--------+--------------------------+--------------------------+---------+---------------------+-------+-----------------------------------------------------------+ 
| 1 | SIMPLE  | posts_tags | index | index_post_id_and_tag_id | index_post_id_and_tag_id | 10  | NULL    | 24159 | Using where; Using index; Using temporary; Using filesort | 
| 1 | SIMPLE  | posts  | eq_ref | PRIMARY     | PRIMARY     | 4  | .posts_tags.post_id |  1 |               | 
+----+-------------+------------+--------+--------------------------+--------------------------+---------+---------------------+-------+-----------------------------------------------------------+ 
2 rows in set (0.00 sec) 

Jakie indeksu trzeba zdefiniować, aby uniknąć mysql używając filesort? Czy jest to możliwe, gdy w polu zamówienia nie ma klauzuli where?

aktualizacja: Profilowanie Wyniki:

 
mysql> show profile for query 1; 
+--------------------------------+----------+ 
| Status       | Duration | 
+--------------------------------+----------+ 
| starting      | 0.000027 | 
| checking query cache for query | 0.037953 | 
| Opening tables     | 0.000028 | 
| System lock     | 0.010382 | 
| Table lock      | 0.023894 | 
| init       | 0.000057 | 
| optimizing      | 0.010030 | 
| statistics      | 0.000026 | 
| preparing      | 0.000018 | 
| Creating tmp table    | 0.128619 | 
| executing      | 0.000008 | 
| Copying to tmp table   | 1.819463 | 
| Sorting result     | 0.001092 | 
| Sending data     | 0.004239 | 
| end       | 0.000012 | 
| removing tmp table    | 0.000885 | 
| end       | 0.000006 | 
| end       | 0.000005 | 
| query end      | 0.000006 | 
| storing result in query cache | 0.000005 | 
| freeing items     | 0.000021 | 
| closing tables     | 0.000013 | 
| logging slow query    | 0.000004 | 
| cleaning up     | 0.000006 | 
+--------------------------------+----------+ 

Update2:

rzeczywistym zapytania (kilka boolean pola, bardziej bezużyteczne indeksuje)

SELECT `posts`.* FROM `posts` INNER JOIN `posts_tags` 
    ON `posts`.id = `posts_tags`.post_id 
    WHERE ((`posts_tags`.tag_id = 7971)) 
     AND (((posts.trashed = 0) 
     AND (`posts`.`published` = 1 
     AND `posts`.`clan_private` = 0)) 
     AND ((`posts_tags`.tag_id = 7971))) 
    ORDER BY created_at DESC LIMIT 0, 10;

Empty set (1.25 sec)

Z out ORDER BY - 0,01s.

 

+----+-------------+------------+--------+-----------------------------------------+-----------------------+---------+---------------------+-------+--------------------------+ 
| id | select_type | table  | type | possible_keys       | key     | key_len | ref     | rows | Extra     | 
+----+-------------+------------+--------+-----------------------------------------+-----------------------+---------+---------------------+-------+--------------------------+ 
| 1 | SIMPLE  | posts_tags | index | index_posts_tags_on_post_id_and_tag_id | index_posts_tags_... | 10  | NULL    | 23988 | Using where; Using index | 
| 1 | SIMPLE  | posts  | eq_ref | PRIMARY,index_posts_on_trashed_and_crea | PRIMARY    | 4  | .posts_tags.post_id |  1 | Using where    | 
+----+-------------+------------+--------+-----------------------------------------+-----------------------+---------+---------------------+-------+--------------------------+ 

ROZWIĄZANIE

1. Zapytanie zaktualizowana "ORDER BY posts_tags.created_at DESC" (dwa niewielkie zmiany w kodzie aplikacji)
2. Index dodania: index_posts_tags_on_created_at.

To wszystko!

Answer 1

Trzeba by trochę denormalize i skopiuj posts.created_at do tabeli post_tags (nazwałem go post_created_at, można nazwać jak chcesz):

CREATE TABLE `posts_tags` (
    `id` int(11) NOT NULL auto_increment, 
    `post_id` int(11) default NULL, 
    `tag_id` int(11) default NULL, 
    `post_created_at` datetime default NULL, 
    `created_at` datetime default NULL, 
    `updated_at` datetime default NULL, 
    PRIMARY KEY (`id`), 
    KEY `index_posts_tags_on_post_id_and_tag_id` (`post_id`,`tag_id`) 
) ENGINE=InnoDB; 

a następnie dodać indeks do posts_tags na

(tag_id, post_created_at) 

które pozwolą kwerendy, aby uzyskać wszystkie posty za znacznikiem, w odpowiedniej kolejności, bez filesort.

Answer 2

swój klucz index_posts_on_created_at jest posortowana rosnąco ale chcesz Wyniki posortowane malejąco

Answer 3

Spróbuj zmienić KEY index_posts_tags_on_post_id_and_tag_id (post_id, tag_id) do Key index_posts_tags_tag_id (tag_id) i odśwież Explain.

Jaka jest dystrybucja tagów z tagami Posts_Tags?