接上一篇文章：

http://www.cnblogs.com/xiaoit/p/4430300.html

1：首先删掉上一篇建立的索引，重新建立一个。

mysql> DROP INDEX idx1 ON tab_index;

Query OK, 55 rows affected (0.01 sec)

Records: 55  Duplicates: 0  Warnings: 0

　　

mysql> ALTER TABLE tab_index

    -> ADD INDEX idx2(age,dte);

Query OK, 55 rows affected (0.01 sec)

Records: 55  Duplicates: 0  Warnings: 0

　　

按照上一篇的测试结论应该是：

(age) 走索引，且索引长度最短
(age,id)走索引，且索引长度最短

(age,dte)走索引，且索引长度最长
(id,age,dte)走索引，且索引长度最长
(id,dte,age)走索引，且索引长度最长
(dte,id,age)走索引，且索引长度最长
(dte,age,id)走索引，且索引长度最长
(age,dte,id)走索引，且索引长度最长
(age,id,dte)走索引，且索引长度最长

重新测试看下：

mysql> EXPLAIN SELECT id,dte,age

    -> FROM tab_index

    -> WHERE age=31;

+----+-------------+-----------+------+---------------+------+---------+-------+------+-------------+

| id | select_type | table     | type | possible_keys | key  | key_len | ref   | rows | Extra       |

+----+-------------+-----------+------+---------------+------+---------+-------+------+-------------+

|  1 | SIMPLE      | tab_index | ref  | idx2          | idx2 | 5       | const |    1 | Using where |

+----+-------------+-----------+------+---------------+------+---------+-------+------+-------------+

1 row in set (0.01 sec)

　　

mysql> EXPLAIN SELECT id,dte,age

    -> FROM tab_index

    -> WHERE age=31 AND id=3;

+----+-------------+-----------+------+---------------+------+---------+-------+------+-------------+

| id | select_type | table     | type | possible_keys | key  | key_len | ref   | rows | Extra       |

+----+-------------+-----------+------+---------------+------+---------+-------+------+-------------+

|  1 | SIMPLE      | tab_index | ref  | idx2          | idx2 | 5       | const |    1 | Using where |

+----+-------------+-----------+------+---------------+------+---------+-------+------+-------------+

1 row in set (0.00 sec)

　　

mysql> EXPLAIN SELECT id,dte,age

    -> FROM tab_index

    -> WHERE age=31 AND dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00' ;

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

| id | select_type | table     | type  | possible_keys | key  | key_len | ref  | rows | Extra       |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

|  1 | SIMPLE      | tab_index | range | idx2          | idx2 | 14      | NULL |    1 | Using where |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

　　

mysql> EXPLAIN SELECT id,dte,age

    -> FROM tab_index

    -> WHERE id=3 AND dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00' AND age=31;

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

| id | select_type | table     | type  | possible_keys | key  | key_len | ref  | rows | Extra       |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

|  1 | SIMPLE      | tab_index | range | idx2          | idx2 | 14      | NULL |    1 | Using where |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

　　

mysql> EXPLAIN SELECT id,dte,age

    -> FROM tab_index

    -> WHERE id=3 AND age=31 AND dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00';

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

| id | select_type | table     | type  | possible_keys | key  | key_len | ref  | rows | Extra       |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

|  1 | SIMPLE      | tab_index | range | idx2          | idx2 | 14      | NULL |    1 | Using where |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

　　

mysql> EXPLAIN SELECT id,dte,age

    -> FROM tab_index

    -> WHERE dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00' AND id=3  AND age=31;

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

| id | select_type | table     | type  | possible_keys | key  | key_len | ref  | rows | Extra       |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

|  1 | SIMPLE      | tab_index | range | idx2          | idx2 | 14      | NULL |    1 | Using where |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

　　

mysql> EXPLAIN SELECT id,dte,age

    -> FROM tab_index

    -> WHERE dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00' AND age=31 AND id=3;

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

| id | select_type | table     | type  | possible_keys | key  | key_len | ref  | rows | Extra       |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

|  1 | SIMPLE      | tab_index | range | idx2          | idx2 | 14      | NULL |    1 | Using where |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

　　

mysql> EXPLAIN SELECT id,dte,age

    -> FROM tab_index

    -> WHERE id=3 AND dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00' AND age=31;

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

| id | select_type | table     | type  | possible_keys | key  | key_len | ref  | rows | Extra       |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

|  1 | SIMPLE      | tab_index | range | idx2          | idx2 | 14      | NULL |    1 | Using where |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

　　

mysql> EXPLAIN SELECT id,dte,age

    -> FROM tab_index

    -> WHERE id=3 AND dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00' AND age=31;

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

| id | select_type | table     | type  | possible_keys | key  | key_len | ref  | rows | Extra       |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

|  1 | SIMPLE      | tab_index | range | idx2          | idx2 | 14      | NULL |    1 | Using where |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

　　

mysql> EXPLAIN SELECT id,dte,age

    -> FROM tab_index

    -> WHERE age=31 AND id=3 AND dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00'  ;

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

| id | select_type | table     | type  | possible_keys | key  | key_len | ref  | rows | Extra       |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

|  1 | SIMPLE      | tab_index | range | idx2          | idx2 | 14      | NULL |    1 | Using where |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

　　

mysql> EXPLAIN SELECT id,dte,age

    -> FROM tab_index

    -> WHERE age=31 AND dte BETWEEN '2011-05-13 00:00:00' AND '2013-05-13 00:00:00' AND id=3 ;

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

| id | select_type | table     | type  | possible_keys | key  | key_len | ref  | rows | Extra       |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

|  1 | SIMPLE      | tab_index | range | idx2          | idx2 | 14      | NULL |    1 | Using where |

+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

　　

结论正确！

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