If an SQL query doesn't specify any ordering, avoid the implicit sort

that happens with MySQL when a "GROUP BY" clause is included. This is a
backend-specific operation, so any other databases requiring similar
encouragement can have a function added to their own backend code.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@9637 bcc190cf-cafb-0310-a4f2-bffc1f526a37

tests/regressiontests/queries/models.py

@@ -6,6 +6,7 @@ import datetime
 import pickle
 import sys
 
+from django.conf import settings
 from django.db import models
 from django.db.models.query import Q, ITER_CHUNK_SIZE
 
@@ -1053,3 +1054,20 @@ FieldError: Infinite loop caused by ordering.
 []
 
 """
+
+if settings.DATABASE_ENGINE == "mysql":
+    __test__["API_TESTS"] += """
+When grouping without specifying ordering, we add an explicit "ORDER BY NULL"
+portion in MySQL to prevent unnecessary sorting.
+
+>>> query = Tag.objects.values_list('parent_id', flat=True).order_by().query
+>>> query.group_by = ['parent_id']
+>>> sql = query.as_sql()[0]
+>>> fragment = "ORDER BY "
+>>> pos = sql.find(fragment)
+>>> sql.find(fragment, pos + 1) == -1
+True
+>>> sql.find("NULL", pos + len(fragment)) == pos + len(fragment)
+True
+
+"""