id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
25470	Django cast DATE field to DATETIME unexpectedly	Qian Xu	nobody	"I posted a question at http://stackoverflow.com/questions/32795047/have-a-perforamance-issue-of-query-date-object-using-django-queryset

I'm using django 1.8.4 writing a webapp. The backend uses MySQL 5.6 (MyISAM). Recently the number of table records reaches 1 million, it will take 1-1.5 seconds to query all distinct record dates. But using MySQL client, it takes less than 0.001 second.

'''Django Code'''

{{{
class Model1(models.Model):
    date = models.DateField(db_index=True)

# benchmark code
db_dates = Model1.objects.dates(""date"", kind=""day"")
}}}

I dumped the django queries. 

{{{
[{u'time': u'0.000', u'sql': u'SET SQL_AUTO_IS_NULL = 0'}, 
 {u'time': u'1.989', u'sql': u""SELECT DISTINCT CAST(DATE_FORMAT(`app1_model1`.`date`, '%Y-%m-%d 00:00:00') AS DATETIME) AS `datefield` FROM `app1_model1` WHERE `app1_model1`.`date` IS NOT NULL ORDER BY `datefield` ASC""}
]
}}}

I noticed that the second query did a type cast. The cast is not a necessary step. It slows down when the amount of records is huge. I pretty sure this is the root cause of the slow. 

The table is created by django. The field is exactly `DATE` field. I'd like to know, why django cast DATE to DATETIME. Is it a bug? 

Currently, I use a workaround:
{{{
db_dates = [dt['date'] for dt in Model1.objects.order_by('date').values('date').distinct()]
}}}"	Cleanup/optimization	new	Database layer (models, ORM)	1.8	Normal				Unreviewed	0	0	0	0	0	0