MySQL中distinct跟group by性能比较[转]

之前看了网上的一些测试，感觉不是很准确，今天亲自测试了一番。得出了结论(仅在个人计算机上测试，可能不全面，仅供参考)

测试过程：

准备一张测试表 

1    CREATE TABLE `test_test` ( 2      `id` int(11) NOT NULL auto_increment, 3      `num` int(11) NOT NULL default '0', 4      PRIMARY KEY  (`id`) 5     ) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ; 

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       建个储存过程向表中插入10W条数据

01    create procedure p_test(pa int(11))  02     begin  03        04      declare max_num int(11) default 100000;  05      declare i int default 0;  06      declare rand_num int;  07        08      select count(id) into max_num from test_test;  09        10      while i < pa do  11              if max_num < 100000 then  12                      select cast(rand()*100 as unsigned) into rand_num;  13                      insert into test_test(num)values(rand_num);  14              end if;  15              set i = i +1;  16      end while;  17     end 

 调用存储过程插入数据

1    call p_test(100000);

 开始测试：（不加索引）

01    select distinct num from test_test;  02    select num from test_test group by num;  03      04    [SQL] select distinct num from test_test; 05    受影响的行: 0 06    时间: 0.078ms 07      08    [SQL]   09    select num from test_test group by num; 10    受影响的行: 0 11    时间: 0.031ms 

 二、num字段上创建索引

1    ALTER TABLE `test_test` ADD INDEX `num_index` (`num`) ;

再次查询

01    select distinct num from test_test;  02    select num from test_test group by num;  03    [SQL] select distinct num from test_test; 04    受影响的行: 0 05    时间: 0.000ms 06      07    [SQL]   08    select num from test_test group by num; 09    受影响的行: 0 10    时间: 0.000ms 

这时候我们发现时间太小了 0.000秒都无法精确了。

我们转到命令行下 测试

01    mysql> set profiling=1; 02    mysql> select distinct(num) from test_test;  03    mysql> select num from test_test group by num; 04    mysql> show profiles; 05    +----------+------------+----------------------------------------+ 06    | Query_ID | Duration   | Query                                  | 07    +----------+------------+----------------------------------------+ 08    |        1 | 0.00072550 | select distinct(num) from test_test    | 09    |        2 | 0.00071650 | select num from test_test group by num | 10    +----------+------------+----------------------------------------+ 

加了索引之后 distinct 比没加索引的 distinct 快了 107倍。

加了索引之后 group by 比没加索引的 group by 快了 43倍。

再来对比 ：distinct  和 group by

不管是加不加索引 group by 都比 distinct 快。因此使用的时候建议选 group by