Re: Django ORM generate inefficient SQL

This is already tracked in #19259.

Le jeudi 21 août 2014 06:49:41 UTC-4, Alex Lebedev a écrit :

Hi, guys!

I have encountered a problem. When I use the following code in django shell:

    from django.contrib.auth.models import Group
    from django.db.models import Count
    print Group.objects.annotate(cnt=Count('user')).values('id', 'cnt').query.sql_with_params()
 
Django ORM generate the following SQL query:

    'SELECT `auth_group`.`id`, COUNT(`auth_user_groups`.`user_id`) AS `cnt` FROM `auth_group` LEFT OUTER JOIN `auth_user_groups` ON ( `auth_group`.`id` = `auth_user_groups`.`group_id` ) GROUP BY `auth_group`.`id`, `auth_group`.`name` ORDER BY NULL'

"auth_group.name" occurs in "group by" statement. But this column isn't represented in "select" statement. Such query is inefficient (expecially for large tables with many columns and rows).

Debuging of Django SQLCompiler ( https://github.com/django/django/blob/stable/1.6.x/django/db/models/sql/compiler.py#L568 ) gives me the following information:
- postgresql:
    "self.query.select" == "self.query.group_by" == "[(u'auth_group', u'id'), (u'auth_group', 'name')]"
    "self.connection.features.allows_group_by_pk" is False
    "len(self.query.get_meta().concrete_fields) == len(self.query.select)" is False
    'auth_group'.'name' appears in result because of "cols = self.query.group_by + having_group_by + select_cols"
- mysql:
    "self.query.select" == "self.query.group_by" == "[(u'auth_group', u'id'), (u'auth_group', 'name')]"
    "self.connection.features.allows_group_by_pk" is True
    "len(self.query.get_meta().concrete_fields) == len(self.query.select)" is False
    'auth_group'.'name' appears in result because of "cols = self.query.group_by + having_group_by + select_cols"

In the same time, the following code (without .values()):

    from django.contrib.auth.models import Group
    from django.db.models import Count
    print Group.objects.annotate(cnt=Count('user')).query.sql_with_params()

gives the right SQL query for mysql (because "len(self.query.get_meta().concrete_fields) == len(self.query.select)" is True):

'SELECT `auth_group`.`id`, `auth_group`.`name`, COUNT(`auth_user_groups`.`user_id`) AS `cnt` FROM `auth_group` LEFT OUTER JOIN `auth_user_groups` ON ( `auth_group`.`id` = `auth_user_groups`.`group_id` ) GROUP BY `auth_group`.`id` ORDER BY NULL'

Is it a bug? Should I create a bug report or something?

Thanks in advance!

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