mysql慢查询优化之从理论和实践说明limit的优点

吾爱主题阅读：107 2024-04-05 14:03:13 评论：0

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：

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 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：

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 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：

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 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)