009--Explain实战详解：key-len分析

1.key-len长度计算参考：http://www.cnblogs.com/gomysql/p/4004244.html

key_len的长度计算公式：
1.varchr(10)变长字段且允许NULL = 10 * ( character set：utf8=3,gbk=2,latin1=1)+1(NULL)+2(变长字段)
2.varchr(10)变长字段且不允许NULL = 10 *( character set：utf8=3,gbk=2,latin1=1)+2(变长字段)
3.char(10)固定字段且允许NULL = 10 * ( character set：utf8=3,gbk=2,latin1=1)+1(NULL)
4.char(10)固定字段且不允许NULL = 10 * ( character set：utf8=3,gbk=2,latin1=1)

2.key-len分析

• 表示索引中使用的字节数，可通过该列计算查询中使用的索引的长度，在不损失精确性的情况下，长度越短越好
  key-len显示的值为索引字段的最大可能长度，并非实际使用长度，即key_len是根据表定义计算而得，不是通过表内检索出的

3.key-len案例

mysql> explain SELECT t.id,t.`name` FROM t1 t ;
+----+-------------+-------+-------+---------------+---------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key           | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+-------------+
|  1 | SIMPLE      | t     | index | NULL          | index_id_name | 37      | NULL |    3 | Using index |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain SELECT t.`name`,t.id FROM t1 t ;
+----+-------------+-------+-------+---------------+---------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key           | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+-------------+
|  1 | SIMPLE      | t     | index | NULL          | index_id_name | 37      | NULL |    3 | Using index |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain SELECT t.id,t.`name` FROM t1 t WHERE t.id = '1' and t.`name` = 'downeyjr_1';
+----+-------------+-------+-------+-----------------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys         | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+-----------------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t     | const | PRIMARY,index_id_name | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+-----------------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT t.id,t.`name` FROM t1 t WHERE t.id = '1' ;
+----+-------------+-------+-------+-----------------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys         | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+-----------------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t     | const | PRIMARY,index_id_name | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+-----------------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT t.`name`,t.id FROM t1 t WHERE t.id = '1' ;
+----+-------------+-------+-------+-----------------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys         | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+-----------------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t     | const | PRIMARY,index_id_name | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+-----------------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT t.id,t.`name` FROM t1 t WHERE t.`name` = 'downeyjr_1';
+----+-------------+-------+-------+---------------+---------------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key           | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+--------------------------+
|  1 | SIMPLE      | t     | index | NULL          | index_id_name | 37      | NULL |    3 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------------+---------+------+------+--------------------------+
1 row in set (0.00 sec)