diff --git a/core/modules/forum/forum.install b/core/modules/forum/forum.install
index 2eebd7f..5ef7170 100644
--- a/core/modules/forum/forum.install
+++ b/core/modules/forum/forum.install
@@ -219,6 +219,7 @@ function forum_schema() {
     ),
     'indexes' => array(
       'forum_topics' => array('nid', 'tid', 'sticky', 'last_comment_timestamp'),
+      'forum_active' => array('last_comment_timestamp'),
     ),
     'foreign keys' => array(
       'tracked_node' => array(
@@ -244,3 +245,11 @@ function forum_schema() {
 function forum_update_last_removed() {
   return 7003;
 }
+
+/**
+ * Implements hook_update_N().
+ */
+function forum_update_8001() {
+  db_add_index('forum_index',  'forum_active', array('last_comment_timestamp'));
+}
+
diff --git a/core/modules/forum/forum.test b/core/modules/forum/forum.test
index c7c3d9c..2a81806 100644
--- a/core/modules/forum/forum.test
+++ b/core/modules/forum/forum.test
@@ -28,6 +28,9 @@ class ForumTestCase extends DrupalWebTestCase {
    */
   function setUp() {
     parent::setUp('taxonomy', 'comment', 'forum');
+    // Verify that index "last_comment_timestamp" has been added to table forum_index.
+    $this->assertTrue(db_index_exists('forum_index', 'forum_active'), t('Verify that index "last_comment_timestamp" has added to table forum_index.'));
+
     // Create users.
     $this->admin_user = $this->drupalCreateUser(array(
       'access administration pages',