Index on code_owner

Code owner rules are queries on its own.

db/schema.rb

@@ -233,7 +233,7 @@ ActiveRecord::Schema.define(version: 20190103140724) do
     t.integer "approvals_required", limit: 2, default: 0, null: false
     t.boolean "code_owner", default: false, null: false
     t.string "name", null: false
-    t.index ["merge_request_id"], name: "index_approval_merge_request_rules_on_merge_request_id", using: :btree
+    t.index ["merge_request_id", "code_owner"], name: "index_approval_merge_request_rules_1", using: :btree
   end
 
   create_table "approval_merge_request_rules_approvals", id: :bigserial, force: :cascade do |t|

ee/db/migrate/20181204031328_create_approval_rules.rb

@@ -15,10 +15,12 @@ class CreateApprovalRules < ActiveRecord::Migration[5.0]
 
     create_table :approval_merge_request_rules, id: :bigserial do |t|
       t.timestamps_with_timezone
-      t.references :merge_request, index: true, foreign_key: { on_delete: :cascade }, null: false
+      t.references :merge_request, index: false, foreign_key: { on_delete: :cascade }, null: false
       t.integer :approvals_required, limit: 2, default: 0, null: false
       t.boolean :code_owner, default: false, null: false
       t.string :name, null: false
+
+      t.index [:merge_request_id, :code_owner], name: 'index_approval_merge_request_rules_1'
     end
   end
 end