mysql之count

两种引擎对count的处理

CREATE TABLE `test` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `name` char(15) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=4 DEFAULT CHARSET=utf8

mysql> explain select count('id') from test;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                        |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

mysql> explain select count('id') from test where id > 1;
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | test  | index | PRIMARY       | PRIMARY | 4       | NULL |    3 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+

myisam保存了记录的总数，直接返回结果, 但是加where条件之后就必须全表扫描了

CREATE TABLE `test` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `name` char(15) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8

mysql> explain select count(*) from test;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | test  | index | NULL          | PRIMARY | 4       | NULL |    3 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+

mysql> explain select count(*) from test where id > 2;
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | test  | range | PRIMARY       | PRIMARY | 4       | NULL |    1 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+

innodb全表扫描了

note: Using index 使用到覆盖索引，就是直接统计索引了，没有再根据索引去找数据