mysql count(*)空表为何会很慢

lef

Count(*)一个“空表”为什么会很慢呢？只有几十行数据为什么select xxx from table limit 1需要300ms？select min(pk) fromtable也会慢？

见下例：

dba@localhost : test 18:14:32> show create table test_hmw\G

*************************** 1. row ***************************

Table: test_hmw

Create Table: CREATE TABLE `test_hmw` (

`mail` varchar(1024)DEFAULT NULL,

`user_id` bigint(20)DEFAULT NULL,

`id` int(10) unsigned NOTNULL AUTO_INCREMENT,

PRIMARY KEY (`id`)

) ENGINE=InnoDB AUTO_INCREMENT=3997636 DEFAULT CHARSET=utf8

1 row in set (0.00 sec)

dba@localhost : test 18:14:45> select count(*) from test_hmw;

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

| count(*) |

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

| 3813472 |

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

1 row in set (0.69 sec)

dba@localhost : test 18:14:56> show status like '%last_query%';

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

| Variable_name |Value |

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

| Last_query_cost | 776677.599000 |

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

1 row in set (0.00 sec)

dba@localhost : test 18:15:14> delete from test_hmw;

Query OK, 3813472 rows affected (15.11 sec)

dba@localhost : test 18:15:56> select count(*) fromtest_hmw;

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

| count(*) |

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

| 0 |

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

1 row in set (0.31 sec)

dba@localhost : test 18:15:59> show status like '%last_query%';

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

| Variable_name |Value |

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

| Last_query_cost | 757879.799000 |

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

1 row in set (0.00 sec)

我们发现count一个空表的cost和删除前是一样的（时间不一样是因为缓存）；其实在Oracle里也存在这个现象，对于select xx from table where rownum<xxx或者select count(*)之类的操作是从起始的block扫描到HMW；而在MySQL里不是HMW，MySQL的多版本是直接存储在表里的，因此MySQL在删除了大量的数据，而purge thread回收前进行上面的操作就会扫描大量的“空”page，因此也就有了这种现象；

既然知道原因在MySQL可以通过重整表的方式再来看看cost:

dba@localhost : test 18:16:01> optimize table test_hmw;

2 rows in set (1.85 sec)

dba@localhost : test 18:16:14> select count(*) fromtest_hmw;

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

| count(*) |

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

| 0 |

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

1 row in set (0.00 sec)

dba@localhost : test 18:16:16> show status like '%last_query%';

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

| Variable_name |Value |

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

| Last_query_cost | 1.199000 |

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

1 row in set (0.00 sec)

是F

》：_____生日快乐我对自己说蜡烛点了寂寞亮了

什么没有

也许放弃才能靠近你不再见你你才会把我想起