mysql的慢查詢：INNER JOIN + ORDER BY原因文件排序

我試圖優化這個查詢：mysql的慢查詢：INNER JOIN + ORDER BY原因文件排序

 
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;

表的大小是38K行，31K和MySQL使用「文件排序」，所以它得到相當慢。我試圖使用不同的索引，沒有運氣。

 
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)

什麼樣的指數，我需要定義，以避免MySQL的使用文件排序的？當訂單字段不在where子句時可能嗎？

更新：分析結果：

 
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：

實時查詢（一些布爾字段，更無用索引）

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;

空集（1.25秒）

With退出訂單 - 0.01秒。

 

+----+-------------+------------+--------+-----------------------------------------+-----------------------+---------+---------------------+-------+--------------------------+ 
| 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    | 
+----+-------------+------------+--------+-----------------------------------------+-----------------------+---------+---------------------+-------+--------------------------+

SOLUTION

查詢更新爲「ORDER BY posts_tags.created_at DESC」（在應用程序代碼的兩個小的變化）
指數升：index_posts_tags_on_created_at。

就這樣！

來源

2010-06-10 Alexander

您將需要一個非規範化位和posts.created_at字段複製到post_tags表（我把它叫做post_created_at，你能說出它怎麼想）：

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;

，然後添加一個索引topost_tags on

(tag_id, post_created_at)

這將允許查詢以正確的順序獲取標記的所有帖子，而不使用文件。

來源

2010-06-10 19:45:27 nathan

謝謝！我更新了查詢使用posts_tags.created_at進行排序（無需知道標記創建日期）並添加了index_posts_tags_on_created_at ...沒有更多文件！:) – Alexander 2010-06-11 06:52:52

是否有可能通知查詢規劃器，不同表中的兩列是相同的，所以你不必明確指定它應該使用posts_tags.created_at而不是posts.created_at？ – sorenbs 2016-11-03 12:47:39

你的關鍵index_posts_on_created_at將按升序排序，但你想要的結果排序降序

來源

2010-06-10 14:38:01 SQLMenace

它使用filesort與ASC和DESC的排序順序:( – Alexander 2010-06-10 15:09:32

嘗試改變KEY index_posts_tags_on_post_id_and_tag_id（post_id，tag_id）向關鍵index_posts_tags_tag_id（tag_id），並重新發布解釋。

帶Posts_Tags的TagID的分佈是什麼？

來源

2010-06-10 14:42:07 Gary

mysql的慢查詢：INNER JOIN + ORDER BY原因文件排序

回答

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