Optimized the cached_db session backend to check if a key exists in the cache first.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17156 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/contrib/sessions/backends/cached_db.py

@@ -28,6 +28,8 @@ class SessionStore(DBStore):
         return data
 
     def exists(self, session_key):
+        if (KEY_PREFIX + session_key) in cache:
+            return True
         return super(SessionStore, self).exists(session_key)
 
     def save(self, must_create=False):

django/contrib/sessions/tests.py

@@ -293,6 +293,11 @@ class CacheDBSessionTests(SessionTestsMixin, TestCase):
 
     backend = CacheDBSession
 
+    def test_exists_searches_cache_first(self):
+        self.session.save()
+        with self.assertNumQueries(0):
+            self.assertTrue(self.session.exists(self.session.session_key))
+
 
 CacheDBSessionWithTimeZoneTests = override_settings(USE_TZ=True)(CacheDBSessionTests)