服务器之家

服务器之家 > 正文

mysql慢查询优化之从理论和实践说明limit的优点

时间:2020-09-20 15:56     来源/作者:stpeace

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)

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)

总结

以上就是这篇文章的全部内容了,希望本文的内容对大家的学习或者工作具有一定的参考学习价值,谢谢大家对服务器之家的支持。如果你想了解更多相关内容请查看下面相关链接

原文链接:https://blog.csdn.net/stpeace/article/details/79123980

相关文章

热门资讯

2020微信伤感网名听哭了 让对方看到心疼的伤感网名大全
2020微信伤感网名听哭了 让对方看到心疼的伤感网名大全 2019-12-26
Intellij idea2020永久破解,亲测可用!!!
Intellij idea2020永久破解,亲测可用!!! 2020-07-29
歪歪漫画vip账号共享2020_yy漫画免费账号密码共享
歪歪漫画vip账号共享2020_yy漫画免费账号密码共享 2020-04-07
电视剧《琉璃》全集在线观看 琉璃美人煞1-59集免费观看地址
电视剧《琉璃》全集在线观看 琉璃美人煞1-59集免费观看地址 2020-08-12
最新idea2020注册码永久激活(激活到2100年)
最新idea2020注册码永久激活(激活到2100年) 2020-07-29
返回顶部