feat: add index to icons table

[taiwork]

こちらのissue
#14

Count: 11156  Time=0.01s (120s)  Lock=0.00s (0s)  Rows=0.7 (8153), isucon[isucon]@localhost
  SELECT image FROM icons WHERE user_id = 'S'

before

mysql> explain SELECT image FROM icons WHERE user_id = 'S';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | icons | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 2 warnings (0.00 sec)

実行

ALTER TABLE `icons` ADD INDEX `idx_user_id` (`user_id`);

after

mysql> explain SELECT image FROM icons WHERE user_id = 'S';
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key         | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | icons | NULL       | ref  | idx_user_id   | idx_user_id | 8       | const |    1 |   100.00 | Using index condition |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-----------------------+
1 row in set, 2 warnings (0.00 sec)