mysql问题集锦1

mysql> desc UserInfoHistory_bak;
+----------------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------------------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| user_id | int(11) | NO | MUL | NULL | |
| user_name | varchar(20) | NO | | NULL | |
| user_last_login_date | datetime | NO | MUL | NULL | |
| user_password | varchar(50) | NO | MUL | NULL | |
+----------------------+-------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

mysql> explain select date(user_last_login_date) as day ,count(user_id) as userNum from UserInfoHistory_bak where date(user_last_login_date) between '2011-07-20' and '2011-07-27' and user_password!='123456' group by date(user_last_login_date);
+----+-------------+---------------------+------+---------------+------+---------+------+---------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------------+------+---------------+------+---------+------+---------+----------------------------------------------+
| 1 | SIMPLE | UserInfoHistory_bak | ALL | NULL | NULL | NULL | NULL | 3227709 | Using where; Using temporary; Using filesort |
+----+-------------+---------------------+------+---------------+------+---------+------+---------+----------------------------------------------+
1 row in set (0.00 sec)

mysql>

mysql> explain select id,user_last_login_date as day ,count(user_id) as userNum from UserInfoHistory_bak where date(user_last_login_date) between '2011-07-20 00:00:00' and '2011-07-27 00:00:00' and user_password!='fetion2010' group by user_last_login_date;
+----+-------------+---------------------+------+---------------------+------+---------+------+---------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------------+------+---------------------+------+---------+------+---------+----------------------------------------------+
| 1 | SIMPLE | UserInfoHistory_bak | ALL | index_user_password | NULL | NULL | NULL | 3227709 | Using where; Using temporary; Using filesort |
+----+-------------+---------------------+------+---------------------+------+---------+------+---------+----------------------------------------------+
1 row in set (0.00 sec)
mysql>

mysql> explain select id,date(user_last_login_date) as day ,count(user_id) as userNum from UserInfoHistory_bak where user_last_login_date between '2011-07-20 00:00:00' and '2011-07-27 00:00:00' and user_password!='fetion2010' group by user_last_login_date;
+----+-------------+---------------------+-------+------------------------------------------------+----------------------------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------------+-------+------------------------------------------------+----------------------------+---------+------+-------+-------------+
| 1 | SIMPLE | UserInfoHistory_bak | range | index_user_last_login_date,index_user_password | index_user_last_login_date | 8 | NULL | 12773 | Using where |
+----+-------------+---------------------+-------+------------------------------------------------+----------------------------+---------+------+-------+-------------+
1 row in set (0.00 sec)
mysql>

如果你在索引列使用函数调用或者更复杂的算术表达式，MySQL就不会使用索引，因为它必须计算出每个数据行的表达式值。