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

3 files changed, 18 insertions, 0 deletions

diff --git a/django/db/backends/__init__.py b/django/db/backends/__init__.py
index 353eb39488..736667d2ff 100644
--- a/django/db/backends/__init__.py
+++ b/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
diff --git a/django/db/backends/mysql/base.py b/django/db/backends/mysql/base.py
index 2fb1988f7a..f6eccb94b7 100644
--- a/django/db/backends/mysql/base.py
+++ b/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
 
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index e30deee29b..8c5ef3355e 100644
--- a/django/db/models/sql/query.py
+++ b/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()