mysql count(*)空表为何会很慢
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 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)
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