Aggregation annotate(Max()) nonoptimal query.

[fckin.spam@…]

I've asked this question in ​django-users and got advice to report.
Here is the point.

Model:

class Home (models.Model): 
    person = models.ForeignKey (Person) 
    state = models.ForeignKey (States) 
    date = models.DateTimeField () 
    host = models.ForeignKey (Hosts) 
    time_spent = models.PositiveIntegerField (null = True)

Here is the expression with query it makes:

>>> print Home.objects.values('person').annotate(Max('id')).order_by().query

SELECT `main_home`.`person_id`, MAX(`main_home`.`id`) AS `id__max` 
FROM `main_home` 
GROUP BY `main_home`.`person_id`, `main_home`.`person_id` 
ORDER BY NULL

GROUP BY formed with double main_home.person_id. Let's see what happen in this case:

mysql> explain SELECT `main_home`.`person_id`, MAX(`main_home`.`id`)  AS `id__max`  FROM `main_home`  GROUP BY `main_home`.`person_id`, `main_home`.`person_id`  ORDER BY NULL;
+----+-------------+-----------+-------+---------------+--------------------+---------+------+------+------------------------------+
| id | select_type | table     | type  | possible_keys | key                | key_len | ref  | rows | Extra                        |
+----+-------------+-----------+-------+---------------+--------------------+---------+------+------+------------------------------+
|  1 | SIMPLE      | main_home | index | NULL          | main_home_21b911c5 | 4       | NULL | 4604 | Using index; Using temporary |
+----+-------------+-----------+-------+---------------+--------------------+---------+------+------+------------------------------+

In "Extra" you can see "Using temporary" option.
Expression execution takes really long time, despite table indexes.

If i'll manually remove main_home.person_id doubling from mysql query, "Using temporary" will disappear and execution runs a times fister.

[anonymous]

Duplicate of #15709.