Fixed #17506 -- Did a minor optimization in the sessions' database backend. Thanks to FunkyBob for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17390 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/contrib/sessions/backends/db.py

@@ -24,11 +24,7 @@ class SessionStore(SessionBase):
             return {}
 
     def exists(self, session_key):
-        try:
-            Session.objects.get(session_key=session_key)
-        except Session.DoesNotExist:
-            return False
-        return True
+        return Session.objects.filter(session_key=session_key).exists()
 
     def create(self):
         while True: