The name index is an index over the last_name and first_name columns. The index can be used for lookups in queries that specify values in a known range for combinations of last_name and first_name values. It can also be used for queries that specify just a last_name value because that column is a leftmost prefix of the index (as described later in this section). Therefore, the name index is used for lookups in the following queries。
既然是最左原则，那么尝试着写一条SQL如下：
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mysql> explain SELECT*FROM org WHERE org_name='1'and org_parent_id=1; +----+-------------+-------+------------+------+---------------+------------+---------+-------------+------+----------+-------------+ | id | select_type |table| partitions | type | possible_keys | key | key_len |ref|rows| filtered | Extra | +----+-------------+-------+------------+------+---------------+------------+---------+-------------+------+----------+-------------+ |1| SIMPLE | org |NULL|ref| index_name | index_name |773| const,const |1|100.00|Using index | +----+-------------+-------+------------+------+---------------+------------+---------+-------------+------+----------+-------------+ 1rowinset, 1 warning (0.01 sec)
可以看到其type是 ref 类型的，于是去mysql官网寻找ref类型的索引是什么：
All rows with matching index values are read from this table for each combination of rows from the previous tables. ref is used if the join uses only a leftmost prefix of the key or if the key is not a PRIMARY KEY or UNIQUE index (in other words, if the join cannot select a single row based on the key value). If the key that is used matches only a few rows, this is a good join type.
mysql> explain SELECT*FROM org_copy WHERE org_name='1'and org_parent_id>1; +----+-------------+----------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+ | id | select_type |table| partitions | type | possible_keys | key | key_len |ref|rows| filtered | Extra | +----+-------------+----------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+ |1| SIMPLE | org_copy |NULL|range| index_name | index_name |773|NULL|1|100.00|Using index condition| +----+-------------+----------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+ 1rowinset, 1 warning (0.00 sec)
可以看到确实是使用了聚合索引，同时最后一列的 Using index condition也表明，这条查询语句会先通过索引来过滤出符合的数据，然后从过滤出来的数据里面在使用我们的where条件进行二次过滤，最终找出符合的数据。
解释
在Mysql里面，索引是以B+树的形式实现的，而索引又分为一级索引(主键)和二级索引(该文章中的index_name)，其实在InnoDB里面，二级索引的叶子节点所存放的数据就是主键索引所对应的地址，这也称之为回表。 回到文章中来，在这篇文章中介绍的index_name索引里面，mysql是根据索引列的顺序，一个一个在索引里面进行查找过滤，如果该索引在某一列断开了，例如索引A的列是(A、B、C)，但是在SQL里面却是where A ='a' and C='c'，此时就会只使用该索引A的A列，这个跟B+树的特点有关。