On Wed, Feb 10, 2010 at 2:22 AM, Russell Keith-Magee <freakboy3...@gmail.com
> wrote:
> It certainly doesn't look right; .exists() should be faster than
> .count(), and having all the extra columns in the query looks a little
> weird.
>
> However, to say for sure, we need a few more details:
>
> * Does this only happen when using David's devserver, or does it
> happen in a normal Django shell?
>
>
I can recreate with just simple shell queries and checking the sql in
django.db.connection
> * Can you give a *specific* example of a model and query that is
> causing this? Or, if it's happening in the process of running a
> management command or other Django-internal activity, what do you need
> to do to stimulate the problem?
>
Every exists() query I tried shows this behavior. For example:
>>> from django.contrib.auth.models import User
>>> User.objects.filter(username='kmt').exists()
True
>>> from django.db import connection
>>> connection.queries[-1]
{'time': '0.001', 'sql': u'SELECT (1) AS `a`, `auth_user`.`id`,
`auth_user`.`username`, `auth_user`.`first_name`, `auth_user`.`last_name`,
`auth_user`.`email`, `auth_user`.`password`, `auth_user`.`is_staff`,
`auth_user`.`is_active`, `auth_user`.`is_superuser`,
`auth_user`.`last_login`, `auth_user`.`date_joined` FROM `auth_user` WHERE
`auth_user`.`username` = kmt '}
>>>
>
> * Are you doing anything interesting with select_related() or
> anything like that?
>
>
No.
> * What version of Django (including SVN revision, if appropriate) are
> you using?
>
I tried current trunk and backed off to the changeset where the function was
added -- query was the same. Looking at the idiom it replaced, the extra
select of the constant 1 value as 'a' was followed by a values('a') that
turned the result into a ValuesQuerySet. I don't see where this is being
done with the exists() implementation. As a result it seems to be a regular
QuerySet with a query with default_cols=True, resulting in all the default
columns getting pulled into the query.
Karen
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