count(*)与count(col)对比

Masql的MyISAM引擎 count(*)与count(col)对比

不带where的条件下:

使用的是Server version: 5.1.34 MySQL Community Server (GPL)

使用count(*)执行结果

mysql> select count(*) from my_cms_25;                 
+----------+
| count(*) |
+----------+
| 1022711 |
+----------+
1 row in set (0.00 sec)

mysql> show profiles;
+----------+------------+--------------------------------+
| Query_ID | Duration   | Query                          |
+----------+------------+--------------------------------+
|        1 | 0.00027000 | select count(*) from my_cms_25 |
+----------+------------+--------------------------------+

使用count(id)执行结果:

mysql> select count(id) from my_cms_25;
+-----------+
| count(id) |
+-----------+
|   1022711 |
+-----------+
1 row in set (0.00 sec)

mysql> show profiles;
+----------+------------+---------------------------------+
| Query_ID | Duration   | Query                           |
+----------+------------+---------------------------------+
|        1 | 0.00027800 | select count(id) from my_cms_25 |
+----------+------------+---------------------------------+
1 row in set (0.00 sec)

为了保证cache,每次执行一条语句,就把mysql重启动.

mysql> select count(x_rank) from my_cms_25;
+---------------+
| count(x_rank) |
+---------------+
|       1022711 |
+---------------+
1 row in set (0.00 sec)

mysql> show profiles;
+----------+------------+-------------------------------------+
| Query_ID | Duration   | Query                               |
+----------+------------+-------------------------------------+
|        1 | 0.00029200 | select count(x_rank) from my_cms_25 |
+----------+------------+-------------------------------------+
1 row in set (0.00 sec)

count(*)比其他两种都快, 不管是count(*), count(id)还是count(x_rank) 执行时间差的不是太多.

注: id是primary key, x_rank是非index.

 

 

带where从句:

mysql> select count(*) from my_cms_25 where id > 100000;
+----------+
| count(*) |
+----------+
|   922711 |
+----------+
1 row in set (0.55 sec)

mysql> show profiles;
+----------+------------+--------------------------------------------------+
| Query_ID | Duration   | Query                                            |
+----------+------------+--------------------------------------------------+
|        1 | 0.54154100 | select count(*) from my_cms_25 where id > 100000 |
+----------+------------+--------------------------------------------------+
1 row in set (0.00 sec)

mysql> select count(id) from my_cms_25 where id > 100000;
+-----------+
| count(id) |
+-----------+
|    922711 |
+-----------+
1 row in set (0.52 sec)

mysql> show profiles;
+----------+------------+---------------------------------------------------+
| Query_ID | Duration   | Query                                             |
+----------+------------+---------------------------------------------------+
|        1 | 0.51955100 | select count(id) from my_cms_25 where id > 100000 |
+----------+------------+---------------------------------------------------+
1 row in set (0.00 sec)

 

mysql> select count(*) from my_cms_25 where id > 100;
+----------+
| count(*) |
+----------+
| 1022611 |
+----------+
1 row in set (0.60 sec)

mysql> show profiles;
+----------+------------+-----------------------------------------------+
| Query_ID | Duration   | Query                                         |
+----------+------------+-----------------------------------------------+
|        1 | 0.59619000 | select count(*) from my_cms_25 where id > 100 |
+----------+------------+-----------------------------------------------+
1 row in set (0.00 sec)

mysql> select count(id) from my_cms_25 where id > 100;
+-----------+
| count(id) |
+-----------+
|   1022611 |
+-----------+
1 row in set (0.57 sec)

mysql> show profiles;
+----------+------------+------------------------------------------------+
| Query_ID | Duration   | Query                                          |
+----------+------------+------------------------------------------------+
|        1 | 0.57422700 | select count(id) from my_cms_25 where id > 100 |
+----------+------------+------------------------------------------------+
1 row in set (0.00 sec)

mysql> select count(x_rank) from my_cms_25 where id > 100;
+---------------+
| count(x_rank) |
+---------------+
|       1022611 |
+---------------+
1 row in set (1.31 sec)

mysql> show profiles;
+----------+------------+----------------------------------------------------+
| Query_ID | Duration   | Query                                              |
+----------+------------+----------------------------------------------------+
|        1 | 1.30890800 | select count(x_rank) from my_cms_25 where id > 100 |
+----------+------------+----------------------------------------------------+
1 row in set (0.00 sec)

带where条件的, 测试的结果count(id) > count(*) > count(x_rank)

就是说,count(*)不一定比count(pk)快.

 

 

对于innodb engine的, 叶兄做了一个测试,自己也做了一个测试,测试结果不相同:

http://imysql.cn/2008_06_24_speedup_innodb_count

http://bugs.mysql.com/bug.php?id=19271

 

使用的版本:Server version: 5.1.30-log Source distribution

mysql> select count(*) from relation;
+----------+
| count(*) |
+----------+
| 3010500 |
+----------+
1 row in set (10.60 sec)

mysql> show profiles;
+----------+-------------+-------------------------------+
| Query_ID | Duration    | Query                         |
+----------+-------------+-------------------------------+
|        1 | 10.59423300 | select count(*) from relation |
+----------+-------------+-------------------------------+
1 row in set (0.01 sec)

 

 

mysql> select count(id) from relation;
+-----------+
| count(id) |
+-----------+
|   3010329 |
+-----------+
1 row in set (3.87 sec)

mysql> show profiles;
+----------+------------+--------------------------------+
| Query_ID | Duration   | Query                          |
+----------+------------+--------------------------------+
|        1 | 3.87004700 | select count(id) from relation |
+----------+------------+--------------------------------+
1 row in set (0.00 sec)

 

 

mysql> select count(*) from relation where id>100;
+----------+
| count(*) |
+----------+
| 3010158 |
+----------+
1 row in set (1.03 sec)

mysql> show profiles;                
+----------+------------+--------------------------------------------+
| Query_ID | Duration   | Query                                      |
+----------+------------+--------------------------------------------+
|        1 | 3.87004700 | select count(id) from relation             |
|        2 | 1.03079800 | select count(*) from relation where id>100 |
+----------+------------+--------------------------------------------+

 

 

mysql> show profiles;
+----------+------------+--------------------------------------------------+
| Query_ID | Duration   | Query                                            |
+----------+------------+--------------------------------------------------+
|        1 | 3.87004700 | select count(id) from relation                   |
|        2 | 1.03079800 | select count(*) from relation where id>100       |
|        3 | 1.01789300 | select count(id) from relation where id>100      |
|        4 | 1.32853100 | select count(*) from relation where infoid > 100 |
+----------+------------+--------------------------------------------------+

 

 

 

 

测试结果不全相同. 不管带不带where条件时count(pk)是比count(*)快的．

count(secondary key) 是没有count(pk)快的。难道bug fix了在5.1.30?

 

感觉应该是pk的存储结构和secondary key不同.这个和官方说的符合,primary key的scan是最快的原因吧.

 

从mysql query optimizer角度上说, count(*) 需要mysql分析更多的column,这个是有一定的overhead的。

但直接count(pk)就没有分析的这个开销.:)

 

下面是更详细,可以看出慢在那一步:
在myisam engine上:

 

 

 

mysql> select count(*) from my_cms_25;
+----------+
| count(*) |
+----------+
| 1022711 |
+----------+
1 row in set (0.00 sec)

mysql> show profile;
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000033 |
| checking query cache for query | 0.000088 |
| Opening tables                 | 0.000014 |
| System lock                    | 0.000005 |
| Table lock                     | 0.000026 |
| init                           | 0.000033 |
| optimizing                     | 0.000013 |
| executing                      | 0.000015 |
| end                            | 0.000004 |
| query end                      | 0.000003 |
| freeing items                  | 0.000024 |
| storing result in query cache | 0.000012 |
| logging slow query             | 0.000003 |
| cleaning up                    | 0.000004 |
+--------------------------------+----------+
14 rows in set (0.00 sec)

mysql> show profiles;
+----------+------------+--------------------------------+
| Query_ID | Duration   | Query                          |
+----------+------------+--------------------------------+
|        1 | 0.00027700 | select count(*) from my_cms_25 |
+----------+------------+--------------------------------+
1 row in set (0.00 sec)

 

 

mysql> select count(id) from my_cms_25;
+-----------+
| count(id) |
+-----------+
|   1022711 |
+-----------+
1 row in set (0.00 sec)

mysql> show profile;
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000056 |
| checking query cache for query | 0.000077 |
| Opening tables                 | 0.000014 |
| System lock                    | 0.000005 |
| Table lock                     | 0.000025 |
| init                           | 0.000047 |
| optimizing                     | 0.000014 |
| executing                      | 0.000015 |
| end                            | 0.000004 |
| query end                      | 0.000003 |
| freeing items                  | 0.000027 |
| storing result in query cache | 0.000011 |
| logging slow query             | 0.000004 |
| cleaning up                    | 0.000003 |
+--------------------------------+----------+
14 rows in set (0.00 sec)

mysql> show profiles;
+----------+------------+---------------------------------+
| Query_ID | Duration   | Query                           |
+----------+------------+---------------------------------+
|        1 | 0.00030500 | select count(id) from my_cms_25 |
+----------+------------+---------------------------------+
1 row in set (0.00 sec)

 

 

innodb engine:

 

mysql> select count(*) from sbtest;
+----------+
| count(*) |
+----------+
| 1000000 |
+----------+
1 row in set (1.98 sec)

mysql> show profile;
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000041 |
| checking query cache for query | 0.000082 |
| Opening tables                 | 0.000015 |
| System lock                    | 0.000004 |
| Table lock                     | 0.000042 |
| init                           | 0.000033 |
| optimizing                     | 0.000012 |
| statistics                     | 0.000014 |
| preparing                      | 0.000010 |
| executing                      | 0.000009 |
| Sending data                   | 1.987284 |
| end                            | 0.000019 |
| query end                      | 0.000010 |
| freeing items                  | 0.000078 |
| storing result in query cache | 0.000016 |
| logging slow query             | 0.000005 |
| cleaning up                    | 0.000008 |
+--------------------------------+----------+
17 rows in set (0.00 sec)

mysql> show profiles;
+----------+------------+-----------------------------+
| Query_ID | Duration   | Query                       |
+----------+------------+-----------------------------+
|        1 | 1.98768200 | select count(*) from sbtest |
+----------+------------+-----------------------------+
1 row in set (0.00 sec)

 

 

 

mysql> select count(id) from sbtest;
+-----------+
| count(id) |
+-----------+
|   1000000 |
+-----------+
1 row in set (1.99 sec)

mysql> show profile;
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000034 |
| checking query cache for query | 0.000079 |
| Opening tables                 | 0.000015 |
| System lock                    | 0.000005 |
| Table lock                     | 0.000042 |
| init                           | 0.000037 |
| optimizing                     | 0.000013 |
| statistics                     | 0.000013 |
| preparing                      | 0.000010 |
| executing                      | 0.000021 |
| Sending data                   | 1.982898 |
| end                            | 0.000021 |
| query end                      | 0.000009 |
| freeing items                  | 0.000089 |
| storing result in query cache | 0.000017 |
| logging slow query             | 0.000006 |
| cleaning up                    | 0.000010 |
+--------------------------------+----------+
17 rows in set (0.00 sec)

mysql> show profiles;
+----------+------------+------------------------------+
| Query_ID | Duration   | Query                        |
+----------+------------+------------------------------+
|        1 | 1.98331900 | select count(id) from sbtest |
+----------+------------+------------------------------+
1 row in set (0.00 sec)