diff --git webform.install webform.install
index c28588a..82cc77f 100644
--- webform.install
+++ webform.install
@@ -310,6 +310,9 @@ function webform_schema() {
       'sid_nid' => array('sid', 'nid'),
     ),
     'primary key' => array('sid'),
+    'indexes' => array(
+      'nid_uid_sid' => array('nid', 'uid', 'sid'),
+    ),
   );
 
   $schema['webform_submitted_data'] = array(
@@ -1293,6 +1296,15 @@ function webform_update_6324() {
 }
 
 /**
+ * Add an index to {webform_submissions}.
+ */
+function webform_update_6325() {
+  $ret = array();
+  db_add_index($ret, 'webform_submissions', 'nid_uid_sid', array('nid', 'uid', 'sid'));
+  return $ret;
+}
+
+/**
  * Recursively delete all files and folders in the specified filepath, then
  * delete the containing folder.
  *