diff --git a/django/contrib/sessions/backends/base.py b/django/contrib/sessions/backends/base.py
index f64ef852cfb..b553fa4d58c 100644
--- a/django/contrib/sessions/backends/base.py
+++ b/django/contrib/sessions/backends/base.py
@@ -121,7 +121,11 @@ class SessionBase(object):
         return self._session.iteritems()
 
     def clear(self):
-        self._session.clear()
+        # To avoid unnecessary persistent storage accesses, we set up the
+        # internals directly (loading data wastes time, since we are going to
+        # set it to an empty dict anyway).
+        self._session_cache = {}
+        self.accessed = True
         self.modified = True
 
     def _get_new_session_key(self):
diff --git a/django/contrib/sessions/backends/cache.py b/django/contrib/sessions/backends/cache.py
index 157d18a586f..dad5117ea30 100644
--- a/django/contrib/sessions/backends/cache.py
+++ b/django/contrib/sessions/backends/cache.py
@@ -42,6 +42,8 @@ class SessionStore(SessionBase):
 
     def delete(self, session_key=None):
         if session_key is None:
+            if self._session_key is None:
+                return
             session_key = self._session_key
         self._cache.delete(session_key)
 
diff --git a/django/contrib/sessions/backends/db.py b/django/contrib/sessions/backends/db.py
index e697bc56b12..0d68f13e68b 100644
--- a/django/contrib/sessions/backends/db.py
+++ b/django/contrib/sessions/backends/db.py
@@ -63,6 +63,8 @@ class SessionStore(SessionBase):
 
     def delete(self, session_key=None):
         if session_key is None:
+            if self._session_key is None:
+                return
             session_key = self._session_key
         try:
             Session.objects.get(session_key=session_key).delete()
diff --git a/django/contrib/sessions/backends/file.py b/django/contrib/sessions/backends/file.py
index 3438b097c4e..1540139710d 100644
--- a/django/contrib/sessions/backends/file.py
+++ b/django/contrib/sessions/backends/file.py
@@ -94,6 +94,8 @@ class SessionStore(SessionBase):
 
     def delete(self, session_key=None):
         if session_key is None:
+            if self._session_key is None:
+                return
             session_key = self._session_key
         try:
             os.unlink(self._key_to_file(session_key))