Hi. I've some question about aggregation annotate functions, like Max.
I don't realy like the queries it makes. Let me explain.
Models:
class Person (models.Model):
name = models.CharField (max_length = 100)
login = models.CharField (max_length = 30)
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 made by it:
>>> 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
Here i try to explain it:
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 | 2188 | Using index; Using temporary |
+----+-------------+-----------+-------+---------------
+--------------------+---------+------+------
+------------------------------+
As you can see, Using temporary is added, it's bacause
`main_home`.`person_id` used in GROUP BY 2 times:
GROUP BY `main_home`.`person_id`, `main_home`.`person_id`
and it makes query to run really long time.
If i'll manually remove one of `main_home`.`person_id` from GROUP BY,
query explain will contain no Using temporary, so execution takes
short time.
Could somebody explain to me - is it a feature? If it is, i really
want to listen reason why. If not - then what? A bug?
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