add an index in mysql

mysql.index.txt

优化前:
+----+-------------+------------+--------+---------------+---------+---------+------------------------+------+----------------------------------------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref                    | rows | Extra                                        |
+----+-------------+------------+--------+---------------+---------+---------+------------------------+------+----------------------------------------------+
|  1 | SIMPLE      | ttb        | ALL    | NULL          | NULL    | NULL    | NULL                   |   10 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | dt         | ALL    | PRIMARY       | NULL    | NULL    | NULL                   |    5 | Using where; Using join buffer               |
|  1 | SIMPLE      | tc         | eq_ref | PRIMARY       | PRIMARY | 4       | tp_db.ttb.topic_id     |    1 |                                              |
|  1 | SIMPLE      | last_reply | eq_ref | PRIMARY       | PRIMARY | 4       | tp_db.tc.last_reply_id |    1 |                                              |
+----+-------------+------------+--------+---------------+---------+---------+------------------------+------+----------------------------------------------+

加索引:
alter table ttb add index tag_id tag_id;

优化后：
+----+-------------+------------+--------+---------------+---------+---------+------------------------+------+----------------------------------------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref                    | rows | Extra                                        |
+----+-------------+------------+--------+---------------+---------+---------+------------------------+------+----------------------------------------------+
|  1 | SIMPLE      | ttb        | ref    | tag_id        | tag_id  | 4       | const                  |    4 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | dt         | eq_ref | PRIMARY       | PRIMARY | 4       | tp_db.ttb.topic_id     |    1 |                                              |
|  1 | SIMPLE      | tc         | eq_ref | PRIMARY       | PRIMARY | 4       | tp_db.ttb.topic_id     |    1 |                                              |
|  1 | SIMPLE      | last_reply | eq_ref | PRIMARY       | PRIMARY | 4       | tp_db.tc.last_reply_id |    1 |                                              |
+----+-------------+------------+--------+---------------+---------+---------+------------------------+------+----------------------------------------------+