Fixed #20950 -- Instantiate OrderedDict() only when needed

The use of OrderedDict (even an empty one) was surprisingly slow. By
initializing OrderedDict only when needed it is possible to save
non-trivial amount of computing time (Model.save() is around 30% faster
for example).

This commit targetted sql.Query only, there are likely other places
which could use similar optimizations.

1 files changed, 2 insertions, 2 deletions

diff --git a/django/db/models/sql/compiler.py b/django/db/models/sql/compiler.py
index 1c6e80b538..c42149e45a 100644
--- a/django/db/models/sql/compiler.py
+++ b/django/db/models/sql/compiler.py
@@ -391,7 +391,7 @@ class SQLCompiler(object):
                     if not distinct or elt in select_aliases:
                         result.append('%s %s' % (elt, order))
                         group_by.append((elt, []))
-            elif get_order_dir(field)[0] not in self.query.extra:
+            elif not self.query._extra or get_order_dir(field)[0] not in self.query._extra:
                 # 'col' is of the form 'field' or 'field1__field2' or
                 # '-field1__field2__field', etc.
                 for table, cols, order in self.find_ordering_name(field,
@@ -987,7 +987,7 @@ class SQLUpdateCompiler(SQLCompiler):
         # We need to use a sub-select in the where clause to filter on things
         # from other tables.
         query = self.query.clone(klass=Query)
-        query.extra = {}
+        query._extra = {}
         query.select = []
         query.add_fields([query.get_meta().pk.name])
         # Recheck the count - it is possible that fiddling with the select