Commit a1cbeb9a authored by Malcolm Tredinnick's avatar Malcolm Tredinnick
Browse files

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
parent 7030ab9a

django/db/backends/__init__.py

+8 −0
Original line number Diff line number Diff line
@@ -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

+8 −0
Original line number Diff line number Diff line
@@ -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

+2 −0
Original line number Diff line number Diff line
@@ -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

+18 −0
Original line number Diff line number Diff line
@@ -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



"""