A query optimizer practice about "force index"

Two tables:
table keywords (keyword_id int, keyword varchar(255));
table adwords (keyword_id int, local_traffic, global_traffic);

mysql> explain select * from keywords left join adwords on keywords.keyword_id = adwords.keyword_id where keyword like 'car%';

+----+-------------+----------+-------+---------------+----------+---------+------------------------------+-------+--------------------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+----------+-------+---------------+----------+---------+------------------------------+-------+--------------------------+

| 1 | SIMPLE | keywords | range | ukeyword | ukeyword | 152 | NULL | 92378 | Using where; Using index |

| 1 | SIMPLE | adwords | ref | PRIMARY | PRIMARY | 8 | keywords.keywords.keyword_id | 10735 | |

+----+-------------+----------+-------+---------------+----------+---------+------------------------------+-------+--------------------------+

mysql> explain select * from keywords left join adwords on keywords.keyword_id = adwords.keyword_id where keyword like 'car%' and local_traffic > 10000;

+----+-------------+----------+--------+------------------+---------+---------+-----------------------------+---------+-------------+
| id | select_type | table    | type   | possible_keys    | key     | key_len | ref                         | rows    | Extra       |
+----+-------------+----------+--------+------------------+---------+---------+-----------------------------+---------+-------------+
|  1 | SIMPLE      | adwords  | ALL    | PRIMARY          | NULL    | NULL    | NULL                        | 1073581 | Using where |
|  1 | SIMPLE      | keywords | eq_ref | PRIMARY,ukeyword | PRIMARY | 8       | keywords.adwords.keyword_id |       1 | Using where |
+----+-------------+----------+--------+------------------+---------+---------+-----------------------------+---------+-------------+

After using local_traffic in the query, seems Mysql doesn't use the proper index (ukeyword) first as expected. Make a change:

mysql> explain select * from keywords force index(ukeyword) left join adwords on keywords.keyword_id = adwords.keyword_id where keyword like 'car%' and local_traffic > 10000;
+----+-------------+----------+-------+---------------+----------+---------+------------------------------+-------+--------------------------+
| id | select_type | table    | type  | possible_keys | key      | key_len | ref                          | rows  | Extra                    |
+----+-------------+----------+-------+---------------+----------+---------+------------------------------+-------+--------------------------+
|  1 | SIMPLE      | keywords | range | ukeyword      | ukeyword | 152     | NULL                         | 92378 | Using where; Using index |
|  1 | SIMPLE      | adwords  | ref   | PRIMARY       | PRIMARY  | 8       | keywords.keywords.keyword_id | 10735 | Using where              |
+----+-------------+----------+-------+---------------+----------+---------+------------------------------+-------+--------------------------+

You see, we are using "force index(xx)" to tell Mysql how to use index to execute the query, the performance has a big jump!