diff --git a/privatemsg.install b/privatemsg.install
index f0479ed..86c949d 100644
--- a/privatemsg.install
+++ b/privatemsg.install
@@ -48,7 +48,8 @@ function privatemsg_schema() {
     'indexes'      => array(
       'list'         => array('uid', 'deleted', 'is_new'),
       'messages'     => array('mid', 'uid'),
-      'participants' => array('thread_id', 'uid', 'deleted')
+      'participants' => array('thread_id', 'uid', 'deleted'),
+      'thread_id_created' => array('thread_id', 'created'),
     ),
   );
 
@@ -499,4 +500,13 @@ function privatemsg_update_6007() {
   db_add_index($ret, 'pm_index', 'participants', array('thread_id', 'uid', 'deleted'));
 
   return $ret;
-}
\ No newline at end of file
+}
+
+/**
+ * Add a new index.
+ */
+function privatemsg_update_6008() {
+  $ret = array();
+  db_add_index($ret, 'pm_index', 'thread_id_created', array('thread_id', 'created'));
+  return $ret;
+}