mysql----索引的设计和使用

mysql> desc city;
+-------------+----------------------+------+-----+-------------------+-----------------------------+
| Field       | Type                 | Null | Key | Default           | Extra                       |
+-------------+----------------------+------+-----+-------------------+-----------------------------+
| city_id     | smallint(5) unsigned | NO   | PRI | NULL              | auto_increment              |
| city        | varchar(50)          | NO   | MUL | NULL              |                             |
| country_id  | smallint(5) unsigned | NO   | MUL | NULL              |                             |
| last_update | timestamp            | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+-------------+----------------------+------+-----+-------------------+-----------------------------+
4 rows in set (0.00 sec)

mysql> explain select * from city where city="Fuzhou";
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | city  | NULL       | ref  | cityname      | cityname | 32      | const |    1 |   100.00 | Using where |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> drop index cityname on city;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc city;

mysql> select * from city where city = 'Fuzhou' or city in ('fuzhou', 'anhui');
Empty set (0.00 sec)