Create index on id instead of stage_id in migration

db/post_migrate/20180212101828_add_tmp_partial_null_index_to_builds.rb

@@ -4,11 +4,11 @@ class AddTmpPartialNullIndexToBuilds < ActiveRecord::Migration
   disable_ddl_transaction!
 
   def up
-    add_concurrent_index(:ci_builds, :stage_id, where: 'stage_id IS NULL',
-                                                name: 'tmp_stage_id_partial_null_index')
+    add_concurrent_index(:ci_builds, :id, where: 'stage_id IS NULL',
+                                          name: 'tmp_id_partial_null_index')
   end
 
   def down
-    remove_concurrent_index_by_name(:ci_builds, 'tmp_stage_id_partial_null_index')
+    remove_concurrent_index_by_name(:ci_builds, 'tmp_id_partial_null_index')
   end
 end

db/post_migrate/20180212102028_remove_tmp_partial_null_index_from_builds.rb

@@ -4,11 +4,11 @@ class RemoveTmpPartialNullIndexFromBuilds < ActiveRecord::Migration
   disable_ddl_transaction!
 
   def up
-    remove_concurrent_index_by_name(:ci_builds, 'tmp_stage_id_partial_null_index')
+    remove_concurrent_index_by_name(:ci_builds, 'tmp_id_partial_null_index')
   end
 
   def down
-    add_concurrent_index(:ci_builds, :stage_id, where: 'stage_id IS NULL',
-                                                name: 'tmp_stage_id_partial_null_index')
+    add_concurrent_index(:ci_builds, :id, where: 'stage_id IS NULL',
+                                          name: 'tmp_id_partial_null_index')
   end
 end