mysql慢查询优化------在可用limit的时候要用limit- 数据库

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)