[英]mysql query scanning all rows
伙計們試圖繞過mysql查詢,以了解為什么它正在掃描表中的所有行
我有2個表topic_entry和topic_user
CREATE TABLE `topic_entry` (
`entry_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`topic_id` bigint(20) unsigned NOT NULL,
`entry_created` datetime NOT NULL,
`entry_object` text,
`level` tinyint(4) NOT NULL DEFAULT '3',
PRIMARY KEY (`entry_id`),
KEY `entry_created` (`entry_created`),
KEY `level` (`level`),
KEY `topic_id_2` (`topic_id`,`entry_id`)
) ENGINE=MyISAM;
CREATE TABLE `topic_user` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`topic_id` bigint(20) unsigned NOT NULL,
`user_id` varchar(100) NOT NULL,
`private` enum('y','n') DEFAULT NULL,
`freq` enum('a','d','w') DEFAULT NULL,
`topic_id_freq` varchar(10) DEFAULT NULL,
`casematch` enum('0','1') DEFAULT '0',
`textmatch` enum('0','1') DEFAULT '0',
`topic_name_case` varchar(100) DEFAULT '',
PRIMARY KEY (`id`),
UNIQUE KEY `topic_id_user_id` (`topic_id`,`user_id`),
KEY `user_id` (`user_id`),
KEY `topic_id_freq` (`topic_id_freq`)
) ENGINE=MyISAM;
這是我要運行的查詢
explain
select te.topic_id,te.entry_id
from topic_entry te
WHERE te.topic_id in (select topic_id
from topic_user where user_id ='xyz')
AND te.level=4
AND te.entry_id > 0
ORDER BY te.entry_id DESC
LIMIT 5;
說明輸出顯示它正在掃描所有行
| 1 | PRIMARY| te | range | PRIMARY,level | PRIMARY | 8 | NULL| **722978** | Using where |
| 2 | DEPENDENT SUBQUERY | topic_user | unique_subquery | topic_id_user_id,user_id | topic_id_user_id | 310 | func,const | 1 | Using index; Using where |
嘗試這個:
EXPLAIN
SELECT te.topic_id,te.entry_id
FROM topic_entry te
JOIN topic_user tu ON te.topic_id = tu.topic_id AND tu.user_id = 'xyz'
WHERE te.level=4
AND te.entry_id > 0
ORDER BY te.entry_id DESC
LIMIT 5;
嘗試更改WHERE子句中的鍵順序:
WHERE te.topic_id in (select topic_id from topic_user where user_id ='xyz') AND te.level=4 AND te.entry_id > 0
至
WHERE te.topic_id in (select topic_id from topic_user where user_id ='xyz') AND te.entry_id > 0 AND te.level=4
我不確定100%(目前無法測試),但可能會有所幫助。
原因是MySQL嘗試無法映射您的密鑰
topic_id, level, entry_id
到KEY topic_id_2
,由於附加字段而失敗。
PS和“ Sly Raskal”具有更好的解決方案,或者您可以嘗試將兩者結合起來
關於什么
explain extended
select te.topic_id,te.entry_id
from topic_entry te LEFT JOIN topic_user tu ON
te.topic_id = tu.topic_id AND tu.user_id = 'xyz'
WHERE 1 = 1
AND te.level=4
AND te.entry_id > 0
ORDER BY te.entry_id DESC
LIMIT 5;
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