Loading... <p><span style="font-size: 16px">在某些情况下,or条件可以避免全表扫描的。<br /></span></p> <h2><strong>1 .where 语句里面如果带有or条件, myisam表能用到索引, innodb不行。</strong></h2> <p><strong><span style="font-size: 13px">1)myisam表:</span></strong></p> <pre class="brush:bash;toolbar:false"> CREATE TABLE IF NOT EXISTS `a` ( `id` int(1) NOT NULL AUTO_INCREMENT, `uid` int(11) NOT NULL, `aNum` char(20) DEFAULT NULL, PRIMARY KEY (`id`), KEY `uid` (`uid`) ) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=6 ;</pre> <pre class="brush:bash;toolbar:false">mysql> explain select * from a where id=1 or uid =2; +----+-------------+-------+-------------+---------------+-------------+---------+------+------+---------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------------+---------------+-------------+---------+------+------+---------------------------------------+ | 1 | SIMPLE | a | index_merge | PRIMARY,uid | PRIMARY,uid | 4,4 | NULL | 2 | Using union(PRIMARY,uid); Using where | +----+-------------+-------+-------------+---------------+-------------+---------+------+------+---------------------------------------+ 1 row in set (0.00 sec)</pre> <p>2)innodb表:</p> <pre class="brush:bash;toolbar:false">CREATE TABLE IF NOT EXISTS `a` ( `id` int(1) NOT NULL AUTO_INCREMENT, `uid` int(11) NOT NULL, `aNum` char(20) DEFAULT NULL, PRIMARY KEY (`id`), KEY `uid` (`uid`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=6 ; mysql> explain select * from a where id=1 or uid =2; +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | a | ALL | PRIMARY,uid | NULL | NULL | NULL | 5 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)</pre> <h2><strong><span style="font-size: 16px">2 .必须所有的or条件都必须是独立索引:</span></strong></h2> <pre class="brush:bash;toolbar:false">+-------+---------------------------------------------------------------------------------------------------------------------- | Table | Create Table +-------+---------------------------------------------------------------------------------------------------------------------- | a | CREATE TABLE `a` ( `id` int(1) NOT NULL AUTO_INCREMENT, `uid` int(11) NOT NULL, `aNum` char(20) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=MyISAM AUTO_INCREMENT=6 DEFAULT CHARSET=latin1 | +-------+---------------------------------------------------------------------------------------------------------------------- 1 row in set (0.00 sec)</pre> <p><span>explain查看:</span></p> <pre class="brush:bash;toolbar:false">mysql> explain select * from a where id=1 or uid =2; +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | a | ALL | PRIMARY | NULL | NULL | NULL | 5 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec)</pre> <p>全表扫描了。</p> <h2><strong><span style="font-size: 14px">3. 用UNION替换OR (适用于索引列)</span></strong></h2> <p> 通常情况下, 用UNION替换WHERE子句中的OR将会起到较好的效果. 对索引列使用OR将造成全表扫描. </p> <p> 注意, 以上规则只针对多个索引列有效. 如果有column没有被索引, 查询效率可能会因为你没有选择OR而降低. </p> <p> 在下面的例子中, LOC_ID 和REGION上都建有索引.<br /> 高效: </p> <pre class="brush:python;toolbar:false">select loc_id , loc_desc , region from location where loc_id = 10 union select loc_id , loc_desc , region from location where region = "melbourne"</pre> <p> 低效: </p> <pre class="brush:python;toolbar:false">select loc_id , loc desc , region from location where loc_id = 10 or region = "melbourne"</pre> <p>如果你坚持要用OR, 那就需要返回记录最少的索引列写在最前面.</p> <h2><span style="font-size: 14px">4. 用in来替换or </span></h2> <p> 这是一条简单易记的规则,但是实际的执行效果还须检验,在oracle8i下,两者的执行路径似乎是相同的. <br />低效: <br />select…. from location where loc_id = 10 or loc_id = 20 or loc_id = 30 <br />高效 <br />select… from location where loc_in in (10,20,30);</p> 最后修改:2021 年 12 月 10 日 10 : 53 AM © 允许规范转载 赞赏 如果觉得我的文章对你有用,请随意赞赏 赞赏作者 支付宝微信