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[英]MySQL performance, inner join, how to avoid Using temporary and filesort
[英]Avoid filesort with INNER JOIN + ORDER BY
我一直在閱讀其他帖子,但我沒有設法解決我的問題。
使用DESC
命令查詢速度慢了 20 倍,我必須改進它。 這是查詢:
SELECT posts.post_id, posts.post_b_id, posts.post_title, posts.post_cont, posts.thumb, posts.post_user, boards.board_title_l, boards.board_title
FROM posts
INNER JOIN follow ON posts.post_b_id = follow.board_id
INNER JOIN boards ON posts.post_b_id = boards.board_id
WHERE follow.user_id =1
ORDER BY posts.post_id DESC
LIMIT 10
這些是表格(更新):
CREATE TABLE IF NOT EXISTS `posts` (
`post_id` int(11) NOT NULL AUTO_INCREMENT,
`post_b_id` int(11) unsigned NOT NULL,
`post_title` varchar(50) COLLATE utf8_bin NOT NULL,
`post_cont` text COLLATE utf8_bin NOT NULL,
`post_mintxt` varchar(255) COLLATE utf8_bin NOT NULL,
`post_type` char(3) COLLATE utf8_bin NOT NULL,
`thumb` varchar(200) COLLATE utf8_bin NOT NULL,
`post_user` varchar(16) COLLATE utf8_bin NOT NULL,
`published` enum('0','1') COLLATE utf8_bin NOT NULL,
`post_ip` varchar(94) COLLATE utf8_bin NOT NULL,
`post_ip_dat` int(11) unsigned NOT NULL,
`post_up` int(10) unsigned NOT NULL DEFAULT '0',
`post_down` int(10) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`post_id`),
KEY `post_b_id` (`post_b_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin AUTO_INCREMENT=405 ;
CREATE TABLE IF NOT EXISTS `boards` (
`board_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`board_title_l` varchar(19) COLLATE utf8_bin NOT NULL,
`board_user_id` int(10) unsigned NOT NULL,
`board_title` varchar(19) COLLATE utf8_bin NOT NULL,
`board_user` varchar(16) COLLATE utf8_bin NOT NULL,
`board_txt` tinyint(1) unsigned NOT NULL,
`board_img` tinyint(1) unsigned NOT NULL,
`board_vid` tinyint(1) unsigned NOT NULL,
`board_desc` varchar(100) COLLATE utf8_bin NOT NULL,
`board_mod_p` tinyint(3) unsigned NOT NULL DEFAULT '0',
`board_ip` varchar(94) COLLATE utf8_bin NOT NULL,
`board_dat_ip` int(11) unsigned NOT NULL,
PRIMARY KEY (`board_id`),
UNIQUE KEY `board_title_l` (`board_title_l`),
KEY `board_user_id` (`board_user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin AUTO_INCREMENT=89 ;
CREATE TABLE IF NOT EXISTS `follow` (
`user_id` int(10) unsigned NOT NULL,
`board_id` int(10) unsigned NOT NULL,
PRIMARY KEY (`user_id`,`board_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
使用默認的ASC
順序,它只使用index 和 where ,而DESC
使用索引、where、臨時和文件排序。
id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE follow ref user_id user_id 4 const 2 100.00 Using index; Using temporary; Using filesort
1 SIMPLE boards eq_ref PRIMARY PRIMARY 4 xxxx.follow.board_id 1 100.00
1 SIMPLE posts ref post_b_id post_b_id 4 xxxx.boards.board_id 3 100.00 Using where
如何使查詢以DESC
順序接收結果,而無需文件排序和臨時。
更新:我做了一個新查詢,沒有臨時或文件排序,但類型:索引,過濾:7340.00。 如果帖子在表格的末尾,幾乎與ASC
訂單一樣快,但如果正在搜索的帖子在開頭,則速度會慢。 所以看起來更好,但這還不夠。
SELECT posts.post_id, posts.post_b_id, posts.post_title, posts.post_cont, posts.thumb, posts.post_user, boards.board_title_l, boards.board_title
FROM posts INNER JOIN boards ON posts.post_b_id = boards.board_id
WHERE posts.post_b_id
IN (
SELECT follow.board_id
FROM follow
WHERE follow.user_id = 1
)
ORDER BY posts.post_id DESC
LIMIT 10
解釋:
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY posts index post_b_id PRIMARY 8 NULL 10 7340.00 Using where
1 PRIMARY boards eq_ref PRIMARY PRIMARY 4 xxxx.posts.post_b_id 1 100.00
2 DEPENDENT SUBQUERY follow eq_ref user_id user_id 8 const,func 1 100.00 Using index
更新:解釋來自dened 答案的查詢:
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY <derived2>ALL NULL NULL NULL NULL 10 100.00
1 PRIMARY posts eq_ref PRIMARY,post_b_id PRIMARY 4 sq.post_id 1 100.00
1 PRIMARY boards eq_ref PRIMARY PRIMARY 4 xxxx.posts.post_b_id 1 100.00
2 DERIVED follow ref PRIMARY PRIMARY 4 1 100.00 Using index; Using temporary; Using filesort
2 DERIVED posts ref post_b_id post_b_id 4 xxxx.follow.board_id 6 100.00 Using index
次數:
Original query no order (ASC): 0.187500 seconds
Original query DESC: 2.812500 seconds
Second query posts at the end (DESC): 0.218750 seconds
Second query posts at the beginning (DESC): 3.293750 seconds
dened's query DESC: 0.421875 seconds
dened's query no order (ASC): 0.323750 seconds
有趣的是,如果我添加ORDER BY ASC
與DESC
一樣慢。
改變表格順序將是一種上帝的方式,但正如我在評論中所說的那樣,我無法做到這一點。
您可以通過將所有過濾工作移到僅訪問索引的子查詢(操作索引通常比操作其他數據快得多)並在最外層查詢中獲取其余數據來幫助 MySQL 優化器:
SELECT posts.post_id,
posts.post_b_id,
posts.post_title,
posts.post_cont,
posts.thumb,
posts.post_user,
boards.board_title_l,
boards.board_title
FROM (SELECT post_id
FROM posts
JOIN follow
ON posts.post_b_id = follow.board_id
WHERE follow.user_id = 1
ORDER BY post_id DESC
LIMIT 10) sq
JOIN posts
ON posts.post_id = sq.post_id
JOIN boards
ON boards.board_id = posts.post_b_id
請注意,我省略ORDER BY posts.post_id DESC
從外部查詢,因為它通常更快的是,而不是使用MySQL查詢排序排序在你的代碼的最終結果(MySQL的經常使用文件排序為)。
PS可以替換的唯一鍵follow
表具有主鍵。
增加sort_buffer_size
參數將增加 MySQL 在使用臨時磁盤文件之前使用的內存量,並且應該會有很大幫助。
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