索引小试

http://www.jb51.net/article/50103.htm  mysql 悲观锁 乐观锁

http://blog.csdn.net/xifeijian/article/details/20313977 mysql  锁

建表

CREATE TABLE IF NOT EXISTS `article` (`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`author_id` INT(10) UNSIGNED NOT NULL,
`category_id` INT(10) UNSIGNED NOT NULL,
`views` INT(10) UNSIGNED NOT NULL,
`comments` INT(10) UNSIGNED NOT NULL,
`title` VARBINARY(255) NOT NULL,
`content` TEXT NOT NULL,
PRIMARY KEY (`id`)
);

插数据

INSERT INTO `article`
(`author_id`, `category_id`, `views`, `comments`, `title`, `content`) VALUES
(1, 1, 1, 1, '1', '1'),
(2, 2, 2, 2, '2', '2'),
(1, 1, 3, 3, '3', '3');

建索引

ALTER TABLE `article` ADD INDEX X ( `category_id` , `comments`, `views` );
ALTER TABLE `article` ADD INDEX Y ( `category_id` , `views` ) ;

ALTER TABLE `article` ADD INDEX Z ( `comments` ) ;

 

查表

EXPLAIN
SELECT author_id
FROM `article`
WHERE comments > 1  AND category_id = 1 
ORDER BY views DESC
LIMIT 1

结果

分析

按照 BTree 索引的工做原理,先排序 category_id,若是遇到相同的 category_id 则再排序 comments,若是遇到相同的 comments 则再排序 views。当 comments 字段在联合索引里处于中间位置时,因comments > 1 条件是一个范围值(所谓 range),MySQL 没法利用索引再对后面的 views 部分进行检索,即 range 类型查询字段后面的索引无效

优化：删除索引X    DROP INDEX X ON article;

执行

EXPLAIN
SELECT author_id
FROM `article`
WHERE comments > 1  AND category_id = 1 
ORDER BY views DESC
LIMIT 1

结果

 

执行

EXPLAIN
SELECT author_id
FROM `article`
WHERE comments > 1
ORDER BY views DESC
LIMIT 1

结果

 

结论 联合索引中包含 <  > 操做时，索引中的部分会失效，把这部分从索引中去掉。

        当存在多个索引时 包含 < 的索引会被忽略