mysql中的count(primary_key)、count(1)、count(*)的区别

表结构如下:

mysql> show create table user\G;
*************************** 1. row ***************************
       Table: user
Create Table: CREATE TABLE `user` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  `pwd` varchar(50) NOT NULL,
  `email` varchar(100) NOT NULL,
  `phone` varchar(30) NOT NULL,
  `sex` enum(‘F‘,‘M‘,‘N‘) NOT NULL DEFAULT ‘N‘,
  `addres` varchar(100) NOT NULL,
  `tag` varchar(100) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=5000003 DEFAULT CHARSET=utf8 COMMENT=‘用户表‘
1 row in set (0.00 sec)
下面做一下explain: 1、count(id)
mysql> select count(id) from user;
+-----------+
| count(id) |
+-----------+
|   5000002 |
+-----------+
1 row in set (1.93 sec)
mysql> explain select count(id) from user;
+----+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows    | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
|  1 | SIMPLE      | user  | index | NULL          | name | 152     | NULL | 4998401 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
1 row in set (0.05 sec)
2、count(1)
mysql> select count(1) from user;
+----------+
| count(1) |
+----------+
|  5000002 |
+----------+
1 row in set (0.90 sec)
mysql> explain select count(1) from user;
+----+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows    | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
|  1 | SIMPLE      | user  | index | NULL          | name | 152     | NULL | 4998401 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)
3、count(*)
mysql> select count(*) from user;
+----------+
| count(*) |
+----------+
|  5000002 |
+----------+
1 row in set (0.87 sec)
mysql> explain select count(*) from user;
+----+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows    | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
|  1 | SIMPLE      | user  | index | NULL          | name | 152     | NULL | 4998401 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)
比较三个查询,explain的结果一模一样,这说明这三个的效率是一样的吗?

再看看下面三个操作,带上where条件 sex=‘F‘,以下三个操作中间均会重启mysql服务。

1、count(id)
mysql> select count(id) from user where sex=‘F‘;
+-----------+
| count(id) |
+-----------+
|   1681259 |
+-----------+
1 row in set (18.87 sec)
mysql> explain select count(id) from user where sex=‘F‘;
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
|  1 | SIMPLE      | user  | ALL  | NULL          | NULL | NULL    | NULL | 4998401 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)
2、count(1)
mysql> select count(1) from user where sex=‘F‘;
+----------+
| count(1) |
+----------+
|  1681259 |
+----------+
1 row in set (4.81 sec)
mysql> explain select count(1) from user where sex=‘F‘;
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
|  1 | SIMPLE      | user  | ALL  | NULL          | NULL | NULL    | NULL | 4998401 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)
3、count(*)
mysql> select count(*) from user where sex=‘F‘;
+----------+
| count(*) |
+----------+
|  1681259 |
+----------+
1 row in set (4.69 sec)
mysql> explain select count(*) from user where sex=‘F‘;
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
|  1 | SIMPLE      | user  | ALL  | NULL          | NULL | NULL    | NULL | 4998401 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)
以上三种查询有一些差别,其中count(id)用时最长,count(*)比count(1)速度要稍微快一点。

两组查询,带条件的都没有使用到索引,扫描了全表;而没有条件的则使用了索引name。

所以在应用中尽量不使用count(*)和count(1),杜绝使用count(primary_key)。

网上有很多资料说

没有主键,count(1)比count(*)快;

有主键的话,count(primary_key)最快,但是在上面的测试中发现,count(primary_key)是最慢的,难道是测试不准确?这个有待验证。

如果表只有一个字段,则count(*)是最快的。

说明:

count(1)中的1并不是指第一个column;

count(*)和count(1)一样,包括对值为NULL的统计;

count(column)不包括对值为NULL的统计,这里的column指的不是primary_key;

本文出自 寄凡,http://www.hblpf.com/?post=55

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mysql中的count(primary_key)、count(1)、count(*)的区别

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