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我正在對具有唯一列的連接進行索引掃描;它聲稱正在檢查大量的行,即使它只查找一行。在mysql查詢計劃中發生意外的索引掃描
這是查詢:
select t.id,
t.twitter_id,
t.screen_name,
t.text
from tweets t
inner join twitter_handle th on th.handle = t.screen_name
order by t.created_at desc
limit 1;
添加/刪除限制條款並不會改變查詢計劃。我希望它會掃描許多等於limit子句中的數字的tweets的created_at索引,然後針對twitter_handle執行eq_ref查找。
根據解釋的查詢計劃,但是,是:
+----+-------------+-------+-------+---------------+-------------+---------+------+--------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+-------------+---------+------+--------+----------------------------------------------+
| 1 | SIMPLE | th | index | NULL | handle | 32 | NULL | 100126 | Using index; Using temporary; Using filesort |
| 1 | SIMPLE | t | ref | screen_name | screen_name | 17 | func | 2 | Using where |
+----+-------------+-------+-------+---------------+-------------+---------+------+--------+----------------------------------------------+
注意號碼在連接檢查的索引掃描和ref = FUNC用於第二表的行的訂購。
這個查詢顯示在我的緩慢的查詢日誌中,我很困惑爲什麼mysql選擇這種方式執行查詢。
的架構這兩個表:
CREATE TABLE `twitter_handle` (
`handle_id` int(11) NOT NULL AUTO_INCREMENT,
`handle` varchar(30) CHARACTER SET ascii NOT NULL,
`twitter_token_id` int(11) DEFAULT NULL,
`name` varchar(255) CHARACTER SET utf8 DEFAULT NULL,
`twitter_user_id` int(11) unsigned DEFAULT NULL,
`location` varchar(100) CHARACTER SET utf8 DEFAULT NULL,
`profile_image_url` varchar(255) CHARACTER SET utf8 DEFAULT NULL,
`followers_count` int(11) DEFAULT NULL,
`twitter_list_id` int(4) DEFAULT NULL,
`last_update` timestamp NULL DEFAULT CURRENT_TIMESTAMP,
`bio` varchar(160) CHARACTER SET utf8 DEFAULT NULL,
PRIMARY KEY (`handle_id`),
UNIQUE KEY `handle` (`handle`),
KEY `twitter_token_id` (`twitter_token_id`),
KEY `twitter_user_id` (`twitter_user_id`)
) ENGINE=InnoDB;
CREATE TABLE `tweets` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`twitter_id` char(15) DEFAULT NULL,
`screen_name` varchar(15) NOT NULL,
`logged_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
`text` char(200) NOT NULL,
`created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
`status` enum('pending','processed','ignored','pending_delete','deleted','pending_tweet','preview') NOT NULL DEFAULT 'pending',
`interaction_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `twitter_id_UNIQUE` (`twitter_id`),
UNIQUE KEY `interaction_id_idx` (`interaction_id`),
UNIQUE KEY `interaction_id` (`interaction_id`,`status`),
KEY `screen_name` (`screen_name`,`created_at`),
KEY `status_2` (`status`,`created_at`),
KEY `created_at_2` (`created_at`)
) ENGINE=InnoDB;
用於表別名和豐富的信息 – 2010-10-06 21:40:17