count(*)、count(1)区别

测试mysql版本为8.0.23

Innodb

测试表结构和数据如下

mysql> desc innodb_test;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int          | NO   | PRI | NULL    | auto_increment |
| name  | varchar(255) | YES  |     | NULL    |                |
+-------+--------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
 
mysql> select * from innodb_test;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
|  4 | NULL |
+----+------+
4 rows in set (0.00 sec)

COUNT(*) SQL执行计划如下(COUNT(1)和COUNT(id)情况一样 )

mysql> EXPLAIN SELECT COUNT(*) FROM innodb_test;
+----+-------------+-------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table       | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | innodb_test | NULL       | index | NULL          | PRIMARY | 4       | NULL |    4 |   100.00 | Using index |
+----+-------------+-------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set (0.00 sec)

但是COUNT(name) 执行计划如下，因为name没有索引，所以也不会用到索引

mysql> EXPLAIN SELECT COUNT(name) FROM innodb_test;
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table       | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | innodb_test | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |   100.00 | NULL  |
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set (0.00 sec)

并且COUNT(name) ,因为id=4的name为null，所以只会查出来三条数据。

mysql> SELECT COUNT(name) FROM innodb_test;
+-------------+
| COUNT(name) |
+-------------+
|           3 |
+-------------+
1 row in set (0.00 sec)

Myisam

测试表结构和数据如下(无任何索引)

mysql> desc myisam_test;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int          | NO   | PRI | NULL    | auto_increment |
| name  | varchar(255) | YES  |     | NULL    |                |
+-------+--------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
 
mysql> select * from myisam_test;
+----+------+
| id | name |
+----+------+
|  1 | a    |
|  2 | b    |
|  3 | c    |
|  4 | NULL |
+----+------+
4 rows in set (0.00 sec)

COUNT(*) SQL执行计划如下(COUNT(1)和COUNT(id)情况一样 )，可以看到这个关键词 Select tables optimized away，因为myisam存储了表的行数

mysql> EXPLAIN SELECT COUNT(*) FROM myisam_test;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                        |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL | NULL     | Select tables optimized away |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
1 row in set (0.00 sec)

COUNT(name) 执行计划如下，因为没有索引，所以查询了全表，同理也只能查到三条数据

mysql> EXPLAIN SELECT COUNT(name) FROM myisam_test;
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table       | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | myisam_test | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |   100.00 | NULL  |
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set (0.00 sec)