diff --git a/modules/system/system.install b/modules/system/system.install
index 43c7383..4df56dc 100644
--- a/modules/system/system.install
+++ b/modules/system/system.install
@@ -1459,7 +1459,11 @@ function system_schema() {
       'value' => array('value'),
       'expire' => array('expire'),
     ),
-    'primary key' => array('name'),
+    'unique keys' => array(
+      'name' => array('name'),
+    ),
+    'primary key' => array('name, value'),
+    'mysql_engine' => 'MEMORY',
   );
 
   $schema['sequences'] = array(