mysql5.6.30之count（*）详解

一：count

统计目标列不为空的数据行。

例：

mysql> select * from t6;
+------+----------+
| id   | name     |
+------+----------+
|    1 | ziOLI    |
|    2 | xiaoguo  |
|    3 | xiaowang |
|    4 |          |
|    5 | NULL     |
+------+----------+

如果是count(*)那么就会统计不为空最多的那个列。统计的数据包含为“”不包含为null的数据。

例：

mysql> select count(name) from t6;
+-------------+
| count(name) |
+-------------+
|           4 |
+-------------+

拓展：count(*)与索引

版本：mysql：5.6.30

引擎：innodb

一： 何时走索引：

创建表test插入测试数据：

mysql> create table test(id int(10),name varchar(10));
Query OK, 0 rows affected (0.02 sec)

mysql> insert into test values(1,"dfd"),(2,"fdfd");
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

无索引查询执行计划：

mysql> explain select count(*) from test\G;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: test
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 2
Extra: NULL
1 row in set (0.00 sec)

添加主键测试count(id)

mysql> alter table test add primary key(id);
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select count(id) from test\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: test
type: index
possible_keys: NULL
key: PRIMARY
key_len: 4
ref: NULL
rows: 2
Extra: Using index
1 row in set (0.00 sec)

结论：走了主键索引

测试count(*)

mysql> explain select count(*) from test\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: test
type: index
possible_keys: NULL
key: PRIMARY
key_len: 4
ref: NULL
rows: 2
Extra: Using index
1 row in set (0.00 sec)

结论：即使我没有指定id字段还是走了主键索引

二：5.6环境下count(*)走主键索引和辅助索引的效率的对比

表结构

mysql> desc sbtest;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type                | Null | Key | Default | Extra          |
+-------+---------------------+------+-----+---------+----------------+
| aid   | bigint(20) unsigned | NO   | PRI | NULL    | auto_increment |
| id    | int(10) unsigned    | NO   | MUL | 0       |                |
| k     | int(10) unsigned    | NO   | MUL | 0       |                |
| c     | char(120)           | NO   |     |         |                |
| pad   | char(60)            | NO   |     |         |                |
+-------+---------------------+------+-----+---------+----------------+

插入1000000行数据

测试：

一：
mysql> explain select count(*) from sbtest;
+----+-------------+--------+-------+---------------+------+---------+------+--------+-------------+
| id | select_type | table  | type  | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+--------+-------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | sbtest | index | NULL          | k    | 4       | NULL | 964496 | Using index |
+----+-------------+--------+-------+---------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)

mysql> select count(*) from sbtest;
+----------+
| count(*) |
+----------+
|  1010101 |
+----------+
1 row in set (0.56 sec)

二：

mysql> explain select count(*) from sbtest where id>=0;
+----+-------------+--------+-------+---------------+------+---------+------+--------+--------------------------+
| id | select_type | table  | type  | possible_keys | key  | key_len | ref  | rows   | Extra                    |
+----+-------------+--------+-------+---------------+------+---------+------+--------+--------------------------+
|  1 | SIMPLE      | sbtest | range | id            | id   | 4       | NULL | 482248 | Using where; Using index |
+----+-------------+--------+-------+---------------+------+---------+------+--------+--------------------------+
1 row in set (0.04 sec)

mysql> select count(*) from sbtest where id>=0;
+----------+
| count(*) |
+----------+
|  1010101 |
+----------+
1 row in set (0.55 sec)

三：
mysql> explain select count(*) from sbtest where aid>=0;
+----+-------------+--------+-------+---------------+---------+---------+------+--------+--------------------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows   | Extra                    |
+----+-------------+--------+-------+---------------+---------+---------+------+--------+--------------------------+
|  1 | SIMPLE      | sbtest | range | PRIMARY       | PRIMARY | 8       | NULL | 482248 | Using where; Using index |
+----+-------------+--------+-------+---------------+---------+---------+------+--------+--------------------------+
1 row in set (0.00 sec)

mysql>  select count(*) from sbtest where aid>=0;
+----------+
| count(*) |
+----------+
|  1010101 |
+----------+
1 row in set (2.52 sec)

部分数据：

aid	id	k	c	pad
1	1	1	dfgfgf	dfdfd
2	2	1	dfgfgf	dfdfd
3	3	1	dfgfgf	dfdfd
4	4	1	dfgfgf	dfdfd

以上测试每次都会重启mysql，因而不存在缓存

结论：在指定条件的情况下主见索引依然没有附属索引快，在不指定的条件的情况下mysql很聪明，选择了走了我数据量较少的索引k。