diff --git a/modules/field/field.install b/modules/field/field.install
index ee8a0a26e2e29b3aadb3d4977e35d6e891753c7d..d19c8f1653b056648a6a3213b7599d6e9c5b803c 100644
--- a/modules/field/field.install
+++ b/modules/field/field.install
@@ -129,10 +129,9 @@ function field_schema() {
       ),
     ),
     'primary key' => array('id'),
-    'unique keys' => array(
-      'field_id_bundle' => array('field_id', 'bundle'),
-    ),
     'indexes' => array(
+      // used by field_delete_instance()
+      'field_name_bundle' => array('field_name', 'bundle'),
       // used by field_read_instances()
       'widget_active_deleted' => array('widget_active', 'deleted'),
       // used by field_modules_disabled()