Fixed select_related performance regressions

The regression was caused by select_related fix for Oracle, commit
c159d9cec0.

django/db/models/options.py

@@ -9,11 +9,10 @@ from django.db.models.fields.related import ManyToManyRel
 from django.db.models.fields import AutoField, FieldDoesNotExist
 from django.db.models.fields.proxy import OrderWrt
 from django.db.models.loading import get_models, app_cache_ready
-from django.utils.translation import activate, deactivate_all, get_language, string_concat
-from django.utils.encoding import force_text, smart_text
-from django.utils.datastructures import SortedDict
 from django.utils import six
-from django.utils.encoding import python_2_unicode_compatible
+from django.utils.datastructures import SortedDict
+from django.utils.encoding import force_text, smart_text, python_2_unicode_compatible
+from django.utils.translation import activate, deactivate_all, get_language, string_concat
 
 # Calculate the verbose_name by converting from InitialCaps to "lowercase with spaces".
 get_verbose_name = lambda class_name: re.sub('(((?<=[a-z])[A-Z])|([A-Z](?![A-Z]|$)))', ' \\1', class_name).lower().strip()
@@ -175,6 +174,10 @@ class Options(object):
             self.pk = field
             field.serialize = False
 
+    def pk_index(self):
+        return [pos for pos, field in enumerate(self.fields)
+                if field == self.pk][0]
+
     def setup_proxy(self, target):
         """
         Does the internal setup so that the current model is a proxy for

django/db/models/query.py

@@ -1395,8 +1395,12 @@ def get_klass_info(klass, max_depth=0, cur_depth=0, requested=None,
                 klass_info = get_klass_info(o.model, max_depth=max_depth, cur_depth=cur_depth+1,
                                             requested=next, only_load=only_load, local_only=True)
                 reverse_related_fields.append((o.field, klass_info))
+    if field_names:
+        pk_idx = field_names.index(klass._meta.pk.attname)
+    else:
+        pk_idx = klass._meta.pk_index()
 
-    return klass, field_names, field_count, related_fields, reverse_related_fields
+    return klass, field_names, field_count, related_fields, reverse_related_fields, pk_idx
 
 
 def get_cached_row(row, index_start, using,  klass_info, offset=0):
@@ -1419,26 +1423,17 @@ def get_cached_row(row, index_start, using,  klass_info, offset=0):
     """
     if klass_info is None:
         return None
-    klass, field_names, field_count, related_fields, reverse_related_fields = klass_info
+    klass, field_names, field_count, related_fields, reverse_related_fields, pk_idx = klass_info
 
     fields = row[index_start : index_start + field_count]
-    # If all the select_related columns are None, then the related
+    # If the pk column is None (or the Oracle equivalent ''), then the related
     # object must be non-existent - set the relation to None.
-    # Otherwise, construct the related object. Also, some backends treat ''
+    if fields[pk_idx] == None or fields[pk_idx] == '':
-    # and None equivalently for char fields, so we have to be prepared for
-    # '' values.
-    if connections[using].features.interprets_empty_strings_as_nulls:
-        vals = tuple([None if f == '' else f for f in fields])
-    else:
-        vals = fields
-
-    if vals == (None,) * field_count:
         obj = None
+    elif field_names:
+        obj = klass(**dict(zip(field_names, fields)))
     else:
-        if field_names:
+        obj = klass(*fields)
-            obj = klass(**dict(zip(field_names, fields)))
-        else:
-            obj = klass(*fields)
 
     # If an object was retrieved, set the database state.
     if obj: