Django cast DATE field to DATETIME unexpectedly

[Qian Xu]

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 = set(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(`model1_table`.`date`, '%Y-%m-%d 00:00:00') AS DATETIME) AS `datefield` FROM `model1_table` WHERE `model1_table`.`date` IS NOT NULL ORDER BY `datefield` ASC"}
]

Actually the second query did a type cast. This is the root cause of the slow.

I'd like to know, why django cast DATE to DATETIME. Is it a bug?