mysql慢查询优化之从理论和实践说明limit的优点

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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