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

django/db/backends/__init__.py

@@ -143,6 +143,14 @@ class BaseDatabaseOperations(object):
         """
         return '%s'
 
+    def force_no_ordering(self):
+        """
+        Returns a list used in the "ORDER BY" clause to force no ordering at
+        all. Returning an empty list means that nothing will be included in the
+        ordering.
+        """
+        return []
+
     def fulltext_search_sql(self, field_name):
         """
         Returns the SQL WHERE clause to use in order to perform a full-text

django/db/backends/mysql/base.py

@@ -133,6 +133,14 @@ class DatabaseOperations(BaseDatabaseOperations):
     def drop_foreignkey_sql(self):
         return "DROP FOREIGN KEY"
 
+    def force_no_ordering(self):
+        """
+        "ORDER BY NULL" prevents MySQL from implicitly ordering by grouped
+        columns. If no ordering would otherwise be applied, we don't want any
+        implicit sorting going on.
+        """
+        return ["NULL"]
+
     def fulltext_search_sql(self, field_name):
         return 'MATCH (%s) AGAINST (%%s IN BOOLEAN MODE)' % field_name
 

django/db/models/sql/query.py

@@ -288,6 +288,8 @@ class BaseQuery(object):
         if self.group_by:
             grouping = self.get_grouping()
             result.append('GROUP BY %s' % ', '.join(grouping))
+            if not ordering:
+                ordering = self.connection.ops.force_no_ordering()
 
         if self.having:
             having, h_params = self.get_having()

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
+
+"""