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mysql慢查询优化
2018-01-22 16:23:44         来源:stpeace的专栏  
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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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