From a1cbeb9afbbf0f16e1ffa1891575fbc2c3edae74 Mon Sep 17 00:00:00 2001
From: Malcolm Tredinnick <malcolm.tredinnick@gmail.com>
Date: Wed, 10 Dec 2008 05:19:27 +0000
Subject: [PATCH] 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          |  8 ++++++++
 django/db/backends/mysql/base.py        |  8 ++++++++
 django/db/models/sql/query.py           |  2 ++
 tests/regressiontests/queries/models.py | 18 ++++++++++++++++++
 4 files changed, 36 insertions(+)

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()
diff --git a/tests/regressiontests/queries/models.py b/tests/regressiontests/queries/models.py
index 868200f60c..d8737df197 100644
--- a/tests/regressiontests/queries/models.py
+++ b/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
+
+"""