Using where：表示优化器需要通过索引回表查询数据；
Using index：表示直接访问索引就足够获取到所需要的数据，不需要通过索引回表；
Using index condition：在5.6版本后加入的新特性（Index Condition Pushdown）;
Using index condition 会先条件过滤索引，过滤完索引后找到所有符合索引条件的数据行，随后用 WHERE 子句中的其他条件去过滤这些数据行；

 using index condition = using index + 回表 + where 过滤

mysql> explain select tid from test where tid < 5 ;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | test  | NULL       | range | idx_tid       | idx_tid | 5       | NULL |   28 |   100.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from test where tid < 5 ;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | test  | NULL       | range | idx_tid       | idx_tid | 5       | NULL |   28 |   100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)