Fixed #18334 -- Fixed detection of supports_stddev backend feature.

Thanks to Michael Manfre for the report and Anssi Kääriäinen for the
review.

django/db/backends/__init__.py

@@ -414,10 +414,11 @@ class BaseDatabaseFeatures(object):
 
     def confirm(self):
         "Perform manual checks of any database features that might vary between installs"
-        self._confirmed = True
-        self.supports_transactions = self._supports_transactions()
-        self.supports_stddev = self._supports_stddev()
-        self.can_introspect_foreign_keys = self._can_introspect_foreign_keys()
+        if not self._confirmed:
+            self._confirmed = True
+            self.supports_transactions = self._supports_transactions()
+            self.supports_stddev = self._supports_stddev()
+            self.can_introspect_foreign_keys = self._can_introspect_foreign_keys()
 
     def _supports_transactions(self):
         "Confirm support for transactions"
@@ -439,8 +440,9 @@ class BaseDatabaseFeatures(object):
 
         try:
             self.connection.ops.check_aggregate_support(StdDevPop())
+            return True
         except NotImplementedError:
-            self.supports_stddev = False
+            return False
 
     def _can_introspect_foreign_keys(self):
         "Confirm support for introspected foreign keys"

tests/regressiontests/backends/tests.py

@@ -397,6 +397,12 @@ class BackendTestCase(TestCase):
         self.assertTrue(hasattr(connection.ops, 'connection'))
         self.assertEqual(connection, connection.ops.connection)
 
+    def test_supports_needed_confirm(self):
+        connection.features.confirm()
+        self.assertIn(connection.features.supports_transactions, (True, False))
+        self.assertIn(connection.features.supports_stddev, (True, False))
+        self.assertIn(connection.features.can_introspect_foreign_keys, (True, False))
+
     def test_duplicate_table_error(self):
         """ Test that creating an existing table returns a DatabaseError """
         cursor = connection.cursor()