Made some types of nested update queries very slightly more efficient at the

database level. Also worked around the fact that MySQL (and maybe other
backends we don't know about) cannot select from the table they're updating.

Fixed #7095.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7496 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/backends/__init__.py

@@ -51,6 +51,7 @@ class BaseDatabaseFeatures(object):
     uses_case_insensitive_names = False
     uses_custom_query_class = False
     empty_fetchmany_value = []
+    update_can_self_select = True
 
 class BaseDatabaseOperations(object):
     """

django/db/backends/mysql/base.py

@@ -63,6 +63,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     autoindexes_primary_keys = False
     inline_fk_references = False
     empty_fetchmany_value = ()
+    update_can_self_select = False
 
 class DatabaseOperations(BaseDatabaseOperations):
     def date_extract_sql(self, lookup_type, field_name):

django/db/backends/mysql_old/base.py

@@ -67,6 +67,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     autoindexes_primary_keys = False
     inline_fk_references = False
     empty_fetchmany_value = ()
+    update_can_self_select = False
 
 class DatabaseOperations(BaseDatabaseOperations):
     def date_extract_sql(self, lookup_type, field_name):

django/db/models/sql/subqueries.py

@@ -159,20 +159,37 @@ class UpdateQuery(Query):
         # from other tables.
         query = self.clone(klass=Query)
         query.bump_prefix()
-        query.select = []
         query.extra_select = {}
-        query.add_fields([query.model._meta.pk.name])
+        first_table = query.tables[0]
+        if query.alias_refcount[first_table] == 1:
+            # We can remove one table from the inner query.
+            query.unref_alias(first_table)
+            for i in xrange(1, len(query.tables)):
+                table = query.tables[i]
+                if query.alias_refcount[table]:
+                    break
+            join_info = query.alias_map[table]
+            query.select = [(join_info[RHS_ALIAS], join_info[RHS_JOIN_COL])]
+            must_pre_select = False
+        else:
+            query.select = []
+            query.add_fields([query.model._meta.pk.name])
+            must_pre_select = not self.connection.features.update_can_self_select
 
         # Now we adjust the current query: reset the where clause and get rid
         # of all the tables we don't need (since they're in the sub-select).
         self.where = self.where_class()
-        if self.related_updates:
+        if self.related_updates or must_pre_select:
+            # Either we're using the idents in multiple update queries (so
+            # don't want them to change), or the db backend doesn't support
+            # selecting from the updating table (e.g. MySQL).
             idents = []
             for rows in query.execute_sql(MULTI):
                 idents.extend([r[0] for r in rows])
             self.add_filter(('pk__in', idents))
             self.related_ids = idents
         else:
+            # The fast path. Filters and updates in one query.
             self.add_filter(('pk__in', query))
         for alias in self.tables[1:]:
             self.alias_refcount[alias] = 0