mysql慢查询优化之从理论和实践说明limit的优点
作者:stpeace
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很多时候, 我们预期查询的结果最多是1条记录数据, 那么这个时候, 最好用上limit 1, 当查到这条数据后, mysql会立即终止继续查询, 不进行更多的无用查询, 从而提升了效率。
我们来实际测试一下, 在一个拥有10万的mysql表中, 查找lily的分数(假设系统中只有1个lily, 而我们预期也只需要这条数据)。为了显示出时间的差别, 我并不对表的name字段建索引。
先看看表结构:
mysql> show create table tb_province; +-------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | tb_province | CREATE TABLE `tb_province` ( `id` bigint(10) unsigned NOT NULL AUTO_INCREMENT, `name` varchar(32) NOT NULL, `score` int(10) unsigned DEFAULT '0', `x` int(10) unsigned DEFAULT '0', `x1` int(10) unsigned DEFAULT '0', `x2` int(10) unsigned DEFAULT '0', `x3` int(10) unsigned DEFAULT '0', `x4` int(10) unsigned DEFAULT '0', `x5` int(10) unsigned DEFAULT '0', `x6` int(10) unsigned DEFAULT '0', `x7` int(10) unsigned DEFAULT '0', `x8` int(10) unsigned DEFAULT '0', `x9` int(10) unsigned DEFAULT '0', `x10` int(10) unsigned DEFAULT '0', PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=124178 DEFAULT CHARSET=latin1 | +-------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)
我们打开set profiling=1;的开关,执行mysql语句来对比:
mysql> select score from tb_province where name='lily'; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.03 sec) mysql> select score from tb_province where name='lily'; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.03 sec) mysql> select score from tb_province where name='lily'; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.04 sec) mysql> select score from tb_province where name='lily'; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.02 sec) mysql> select score from tb_province where name='lily'; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.03 sec) mysql> select score from tb_province where name='lily' limit 1; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.00 sec) mysql> select score from tb_province where name='lily' limit 1; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.00 sec) mysql> select score from tb_province where name='lily' limit 1; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.00 sec) mysql> select score from tb_province where name='lily' limit 1; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.01 sec) mysql> select score from tb_province where name='lily' limit 1; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.00 sec)
可见,我们针对是否采用limit 1进行了5次对比测试, 来看看结果吧:
mysql> show profiles; +----------+------------+---------------------------------------------------------+ | Query_ID | Duration | Query | +----------+------------+---------------------------------------------------------+ | 5 | 0.02686000 | select score from tb_province where name='lily' | | 6 | 0.02649050 | select score from tb_province where name='lily' | | 7 | 0.03413500 | select score from tb_province where name='lily' | | 8 | 0.02601350 | select score from tb_province where name='lily' | | 9 | 0.02785775 | select score from tb_province where name='lily' | | 10 | 0.00042300 | select score from tb_province where name='lily' limit 1 | | 11 | 0.00043250 | select score from tb_province where name='lily' limit 1 | | 12 | 0.00044350 | select score from tb_province where name='lily' limit 1 | | 13 | 0.00053200 | select score from tb_province where name='lily' limit 1 | | 14 | 0.00043250 | select score from tb_province where name='lily' limit 1 | +----------+------------+---------------------------------------------------------+ 14 rows in set, 1 warning (0.00 sec)
可见,采用limit 1后, mysql语句的效率确实提升很多。 当表更大时, 效率提升会更加明显。
我们已经从理论和实践的脚本都说明了limit的优点, 所以, 建议是:在可用limit的时候要用limit (当然, 如果结果是多个,肯定不能limit 1啊)
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