Add index to observation views

[nimmolo]

Frequent queries of observation_views are

• by :observation_id, for the show obs footer, or
• by both :observation_id and :user_id, for the identify query.

In this case, we can use a single index on two columns for both queries, but the order matters.

---

Without index:

irb(main):001> ObservationView.where(observation_id: 81188).explain
  TRANSACTION (0.1ms)  BEGIN
  ObservationView Load (146.7ms)  SELECT `observation_views`.* FROM `observation_views` WHERE `observation_views`.`observation_id` = 81188
=> 
EXPLAIN SELECT `observation_views`.* FROM `observation_views` WHERE `observation_views`.`observation_id` = 81188
+----+-------------+-------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table             | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | observation_views | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 440629 |     10.0 | Using where |
+----+-------------+-------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set (0.00 sec)

irb(main):004> ObservationView.where(user_id: 3477).where(observation_id: 81188).explain
  ObservationView Load (154.4ms)  SELECT `observation_views`.* FROM `observation_views` WHERE (`observation_views`.`user_id` = 3477) AND (`observation_views`.`observation_id` = 81188)
=> 
EXPLAIN SELECT `observation_views`.* FROM `observation_views` WHERE (`observation_views`.`user_id` = 3477) AND (`observation_views`.`observation_id` = 81188)
+----+-------------+-------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table             | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | observation_views | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 440629 |      1.0 | Using where |
+----+-------------+-------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set (0.00 sec)

---

With index on [:observation_id, :user_id]:

irb(main):001> ObservationView.where(user_id: 3477).where(observation_id: 81188).explain
  TRANSACTION (0.1ms)  BEGIN
  ObservationView Load (0.5ms)  SELECT `observation_views`.* FROM `observation_views` WHERE (`observation_views`.`user_id` = 3477) AND (`observation_views`.`observation_id` = 81188)
=> 
EXPLAIN SELECT `observation_views`.* FROM `observation_views` WHERE (`observation_views`.`user_id` = 3477) AND (`observation_views`.`observation_id` = 81188)
+----+-------------+-------------------+------------+------+------------------------+------------------------+---------+-------------+------+----------+-------+
| id | select_type | table             | partitions | type | possible_keys          | key                    | key_len | ref         | rows | filtered | Extra |
+----+-------------+-------------------+------------+------+------------------------+------------------------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | observation_views | NULL       | ref  | user_observation_index | user_observation_index | 10      | const,const |    1 |    100.0 | NULL  |
+----+-------------+-------------------+------------+------+------------------------+------------------------+---------+-------------+------+----------+-------+
1 row in set (0.00 sec)

irb(main):002> ObservationView.where(observation_id: 81188).explain
  ObservationView Load (0.5ms)  SELECT `observation_views`.* FROM `observation_views` WHERE `observation_views`.`observation_id` = 81188
=> 
EXPLAIN SELECT `observation_views`.* FROM `observation_views` WHERE `observation_views`.`observation_id` = 81188
+----+-------------+-------------------+------------+------+------------------------+------------------------+---------+-------+------+----------+-------+
| id | select_type | table             | partitions | type | possible_keys          | key                    | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------------------+------------+------+------------------------+------------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | observation_views | NULL       | ref  | user_observation_index | user_observation_index | 5       | const |    2 |    100.0 | NULL  |
+----+-------------+-------------------+------------+------+------------------------+------------------------+---------+-------+------+----------+-------+
1 row in set (0.00 sec)

---

With index on [:user_id, :observation_id]:

irb(main):001> ObservationView.where(user_id: 3477).where(observation_id: 81188).explain
  TRANSACTION (0.1ms)  BEGIN
  ObservationView Load (0.4ms)  SELECT `observation_views`.* FROM `observation_views` WHERE (`observation_views`.`user_id` = 3477) AND (`observation_views`.`observation_id` = 81188)
=> 
EXPLAIN SELECT `observation_views`.* FROM `observation_views` WHERE (`observation_views`.`user_id` = 3477) AND (`observation_views`.`observation_id` = 81188)
+----+-------------+-------------------+------------+------+------------------------+------------------------+---------+-------------+------+----------+-------+
| id | select_type | table             | partitions | type | possible_keys          | key                    | key_len | ref         | rows | filtered | Extra |
+----+-------------+-------------------+------------+------+------------------------+------------------------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | observation_views | NULL       | ref  | user_observation_index | user_observation_index | 10      | const,const |    1 |    100.0 | NULL  |
+----+-------------+-------------------+------------+------+------------------------+------------------------+---------+-------------+------+----------+-------+
1 row in set (0.00 sec)

irb(main):002> ObservationView.where(observation_id: 81188).explain
  ObservationView Load (148.8ms)  SELECT `observation_views`.* FROM `observation_views` WHERE `observation_views`.`observation_id` = 81188
=> 
EXPLAIN SELECT `observation_views`.* FROM `observation_views` WHERE `observation_views`.`observation_id` = 81188
+----+-------------+-------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table             | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | observation_views | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 440629 |     10.0 | Using where |
+----+-------------+-------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set (0.00 sec)

In this last case, the two-column index is not usable for :observation_id, because it comes second.

[coveralls]

coverage: 94.517%. remained the same
when pulling 3ad1f73 on nimmo-add-target-index-to-comments
into 0c41b7b on main.