mysql distinct跟group by性能

mysql distinct和group by性能

1，测试前的准备

查看复制打印?

//准备一张测试表

mysql> CREATE TABLE `test_test` (

-> `id` int(11) NOT NULL auto_increment,

-> `num` int(11) NOT NULL default '0',

-> PRIMARY KEY (`id`)

-> ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

Query OK, 0 rows affected (0.05 sec)

mysql> delimiter || //改变mysql命令结束符为||

//建个储存过程向表中插入10W条数据

mysql> create procedure p_test(pa int(11))

-> begin

->

-> declare max_num int(11) default 100000;

-> declare i int default 0;

-> declare rand_num int;

->

-> select count(id) into max_num from test_test;

->

-> while i < pa do

-> if max_num < 100000 then

-> select cast(rand()*100 as unsigned) into rand_num;

-> insert into test_test(num)values(rand_num);

-> end if;

-> set i = i +1;

-> end while;

-> end||

Query OK, 0 rows affected (0.00 sec)

mysql> call p_test(100000)||

Query OK, 1 row affected (5.66 sec)

mysql> delimiter ;//改变mysql命令结束符为;

mysql> select count(id) from test_test; //数据都进去了

+-----------+

| count(id) |

+-----------+

| 100000 |

+-----------+

1 row in set (0.00 sec)

mysql> show variables like "%pro%"; //查看一下，记录执行的profiling是不是开启动了，默认是不开启的

+---------------------------+-------+

| Variable_name | Value |

+---------------------------+-------+

| profiling | OFF |

| profiling_history_size | 15 |

| protocol_version | 10 |

| slave_compressed_protocol | OFF |

+---------------------------+-------+

4 rows in set (0.00 sec)

mysql> set profiling=1; //开启

Query OK, 0 rows affected (0.00 sec)

//准备一张测试表
mysql> CREATE TABLE `test_test` (
->   `id` int(11) NOT NULL auto_increment,
->   `num` int(11) NOT NULL default '0',
->   PRIMARY KEY  (`id`)
-> ) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;
Query OK, 0 rows affected (0.05 sec)

mysql> delimiter ||  //改变mysql命令结束符为||

//建个储存过程向表中插入10W条数据
mysql> create procedure p_test(pa int(11))
-> begin
->
->  declare max_num int(11) default 100000;
->  declare i int default 0;
->  declare rand_num int;
->
->  select count(id) into max_num from test_test;
->
->  while i < pa do
->          if max_num < 100000 then
->                  select cast(rand()*100 as unsigned) into rand_num;
->                  insert into test_test(num)values(rand_num);
->          end if;
->          set i = i +1;
->  end while;
-> end||
Query OK, 0 rows affected (0.00 sec)

mysql> call p_test(100000)||
Query OK, 1 row affected (5.66 sec)

mysql> delimiter ;//改变mysql命令结束符为;
mysql> select count(id) from test_test;  //数据都进去了
+-----------+
| count(id) |
+-----------+
|    100000 |
+-----------+
1 row in set (0.00 sec)

mysql> show variables like "%pro%";   //查看一下，记录执行的profiling是不是开启动了，默认是不开启的
+---------------------------+-------+
| Variable_name             | Value |
+---------------------------+-------+
| profiling                 | OFF   |
| profiling_history_size    | 15    |
| protocol_version          | 10    |
| slave_compressed_protocol | OFF   |
+---------------------------+-------+
4 rows in set (0.00 sec)

mysql> set profiling=1;           //开启
Query OK, 0 rows affected (0.00 sec)

2，测试

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//做了4组测试

mysql> select distinct(num) from test_test;

mysql> select num from test_test group by num;

mysql> show profiles; //查看结果

+----------+------------+-------------------------------------------+

| Query_ID | Duration | Query |

+----------+------------+-------------------------------------------+

| 1 | 0.07298225 | select distinct(num) from test_test |

| 2 | 0.07319975 | select num from test_test group by num |

| 3 | 0.07313525 | select num from test_test group by num |

| 4 | 0.07317725 | select distinct(num) from test_test |

| 5 | 0.07275200 | select distinct(num) from test_test |

| 6 | 0.07298600 | select num from test_test group by num |

| 7 | 0.07500700 | select num from test_test group by num |

| 8 | 0.07331325 | select distinct(num) from test_test |

| 9 | 0.57831575 | create index num_index on test_test (num) | //在这儿的时候，我加了索引

| 10 | 0.00243550 | select distinct(num) from test_test |

| 11 | 0.00121975 | select num from test_test group by num |

| 12 | 0.00116550 | select distinct(num) from test_test |

| 13 | 0.00107650 | select num from test_test group by num |

+----------+------------+-------------------------------------------+

13 rows in set (0.00 sec)

//做了4组测试
mysql> select distinct(num) from test_test;
mysql> select num from test_test group by num;

mysql> show profiles;    //查看结果
+----------+------------+-------------------------------------------+
| Query_ID | Duration   | Query                                     |
+----------+------------+-------------------------------------------+
|        1 | 0.07298225 | select distinct(num) from test_test       |
|        2 | 0.07319975 | select num from test_test group by num    |
|        3 | 0.07313525 | select num from test_test group by num    |
|        4 | 0.07317725 | select distinct(num) from test_test       |
|        5 | 0.07275200 | select distinct(num) from test_test       |
|        6 | 0.07298600 | select num from test_test group by num    |
|        7 | 0.07500700 | select num from test_test group by num    |
|        8 | 0.07331325 | select distinct(num) from test_test       |
|        9 | 0.57831575 | create index num_index on test_test (num) |  //在这儿的时候，我加了索引
|       10 | 0.00243550 | select distinct(num) from test_test       |
|       11 | 0.00121975 | select num from test_test group by num    |
|       12 | 0.00116550 | select distinct(num) from test_test       |
|       13 | 0.00107650 | select num from test_test group by num    |
+----------+------------+-------------------------------------------+
13 rows in set (0.00 sec)

上面的1-8是4组数据，并且是没有加索引的，从中我们可以看出，distinct比group by 会好一点点

10-13是2组数据，是加了索引以后的，从中我们可以看出，group by 比distinct 会好一点点

一般情况，数据量比较大的表，关联字段都会加索引的，，并且加索引后检索时间只有以前的六分之一左右。

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