mysql5.6.19下子查询为什么无法使用索引-Mysql Tutorial-php.cn

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mysql5.6.19下子查询为什么无法使用索引

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Jun 07, 2016 pm 04:28 PM

WhyuseInquireindex

表结构很简单 CREATE TABLE `oplogs` (`id` int(10) unsigned NOT NULL AUTO_INCREMENT,`kind` varchar(45) NOT NULL DEFAULT '',`op` varchar(100) NOT NULL,`user` varchar(25) NOT NULL DEFAULT '',`ip` varchar(16) NOT NULL DEFAULT '',`updatetime` ti

表结构很简单

CREATE TABLE `oplogs` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`kind` varchar(45) NOT NULL DEFAULT '',
`op` varchar(100) NOT NULL,
`user` varchar(25) NOT NULL DEFAULT '',
`ip` varchar(16) NOT NULL DEFAULT '',
`updatetime` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=34896 DEFAULT CHARSET=utf8 ROW_FORMAT=FIXED

结果有34895条

sql查询语句为：

SELECT a.id,a.kind,a.op,a.user,a.ip,a.updatetime FROM oplogs as a inner join(select id from oplogs where 1 order by id desc limit 0,20) as b using(id)

在5.6.19情况下

用php读取内容并显示

首次显示结果需要3.5秒

但是同样的配置

在linux 下 5.5.38下

只需要0.7秒左右

很奇怪的情况

然后在5.6下执行了explain 结果如下：

而同样的数据 explain 结构如下：

比较结果很明显主要在于对oplogs进行排序时，rows行变化太大了，一个进行了完整的遍历，第一个使用了索引，造成差距过大，不过原因尚未找到，有哪位知道呢？

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