為什么mysql 走同樣的索引耗時(shí)差別卻巨大?

耗時(shí)查詢：

mysql> select * from channel_details where channel like "%" and type=29 and `year`>=2017 limit 10;
+---------+------+-------+---------------+-------------+---------+------+-----+---------------------+
| id      | year | month | point_of_time | channel     | ratings | type | age | time_at             |
+---------+------+-------+---------------+-------------+---------+------+-----+---------------------+
| 7124305 | 2017 |     1 | 02:00         | ??????????? |       0 |   29 |  40 | 2017-02-15 15:03:54 |
| 7124306 | 2017 |     1 | 02:01         | ??????????? |       0 |   29 |  40 | 2017-02-15 15:03:54 |
| 7124307 | 2017 |     1 | 02:02         | ??????????? |       0 |   29 |  40 | 2017-02-15 15:03:54 |
| 7124308 | 2017 |     1 | 02:03         | ??????????? |       0 |   29 |  40 | 2017-02-15 15:03:54 |
| 7124309 | 2017 |     1 | 02:04         | ??????????? |       0 |   29 |  40 | 2017-02-15 15:03:54 |
| 7124310 | 2017 |     1 | 02:05         | ??????????? |       0 |   29 |  40 | 2017-02-15 15:03:54 |
| 7124311 | 2017 |     1 | 02:06         | ??????????? |       0 |   29 |  40 | 2017-02-15 15:03:54 |
| 7124312 | 2017 |     1 | 02:07         | ??????????? |       0 |   29 |  40 | 2017-02-15 15:03:54 |
| 7124313 | 2017 |     1 | 02:08         | ??????????? |   0.001 |   29 |  40 | 2017-02-15 15:03:54 |
| 7124314 | 2017 |     1 | 02:09         | ??????????? |   0.001 |   29 |  40 | 2017-02-15 15:03:54 |
+---------+------+-------+---------------+-------------+---------+------+-----+---------------------+
10 rows in set (16.10 sec)

mysql> explain select * from channel_details where channel like "%" and type=29 and `year`>=2017 limit 10;
+----+-------------+-----------------+------------+------+---------------+------+---------+-------+---------+----------+-------------+
| id | select_type | table           | partitions | type | possible_keys | key  | key_len | ref   | rows    | filtered | Extra       |
+----+-------------+-----------------+------------+------+---------------+------+---------+-------+---------+----------+-------------+
|  1 | SIMPLE      | channel_details | NULL       | ref  | year,type     | type | 4       | const | 4969150 |     5.56 | Using where |
+----+-------------+-----------------+------------+------+---------------+------+---------+-------+---------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

不耗時(shí)查詢：

mysql> explain select * from channel_details where channel like "%" and type=29  limit 10;
+----+-------------+-----------------+------------+------+---------------+------+---------+-------+---------+----------+-------------+
| id | select_type | table           | partitions | type | possible_keys | key  | key_len | ref   | rows    | filtered | Extra       |
+----+-------------+-----------------+------------+------+---------------+------+---------+-------+---------+----------+-------------+
|  1 | SIMPLE      | channel_details | NULL       | ref  | type          | type | 4       | const | 4969150 |    11.11 | Using where |
+----+-------------+-----------------+------------+------+---------------+------+---------+-------+---------+----------+-------------+
1 row in set, 1 warning (0.09 sec)

mysql> select * from channel_details where channel like "%" and type=29  limit 10;
+----+------+-------+---------------+-------------+---------+------+-----+---------------------+
| id | year | month | point_of_time | channel     | ratings | type | age | time_at             |
+----+------+-------+---------------+-------------+---------+------+-----+---------------------+
|  1 | 2016 |     9 | 02:00         | ??????????? |       0 |   29 |  40 | 2016-12-22 22:59:24 |
|  2 | 2016 |     9 | 02:01         | ??????????? |       0 |   29 |  40 | 2016-12-22 22:59:24 |
|  3 | 2016 |     9 | 02:02         | ??????????? |       0 |   29 |  40 | 2016-12-22 22:59:24 |
|  4 | 2016 |     9 | 02:03         | ??????????? |       0 |   29 |  40 | 2016-12-22 22:59:24 |
|  5 | 2016 |     9 | 02:04         | ??????????? |       0 |   29 |  40 | 2016-12-22 22:59:24 |
|  6 | 2016 |     9 | 02:05         | ??????????? |       0 |   29 |  40 | 2016-12-22 22:59:24 |
|  7 | 2016 |     9 | 02:06         | ??????????? |       0 |   29 |  40 | 2016-12-22 22:59:24 |
|  8 | 2016 |     9 | 02:07         | ??????????? |       0 |   29 |  40 | 2016-12-22 22:59:24 |
|  9 | 2016 |     9 | 02:08         | ??????????? |       0 |   29 |  40 | 2016-12-22 22:59:24 |
| 10 | 2016 |     9 | 02:09         | ??????????? |       0 |   29 |  40 | 2016-12-22 22:59:24 |
+----+------+-------+---------------+-------------+---------+------+-----+---------------------+
10 rows in set (0.11 sec)

明明都是走了type索引，為什么時(shí)間差別這么大？