mysql Cardinality

show index 有一个重要的字段信息 Cardinality,表示索引的散列：
show index from message_message

-------+---------+
| Table           | Non_unique | Key_name           | Seq_in_index | Column_name           | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-----------------+------------+--------------------+--------------+-----------------------+-----------+-------------+----------+--------+------+------------+---------+
| message_message |          0 | PRIMARY            |            1 | id                    | A         |    12171545 |     NULL | NULL   |      | BTREE      |         |
| message_message |          1 | idx_parent         |            1 | parent_id             | A         |     3042886 |     NULL | NULL   | YES  | BTREE      |         |
| message_message |          1 | idx_last_replied   |            1 | last_replied_datetime | A         |    12171545 |     NULL | NULL   |      | BTREE      |         |
| message_message |          1 | idx_x8             |            1 | sender_id             | A         |      338098 |     NULL | NULL   | YES  | BTREE      |         |
| message_message |          1 | idx_x8             |            2 | add_datetime          | A         |    12171545 |     NULL | NULL   |      | BTREE      |         |
| message_message |          1 | idx_cat_add        |            1 | category              | A         |          17 |     NULL | NULL   |      | BTREE      |         |

参考：http://oliver-peng.iteye.com/blog/671309
http://blog.donews.com/maverick/archive/2007/09/28/1214300.aspx
http://www.penglixun.com/tech/database/mysql_show_index_cardinality.html