Add migration with index

db/migrate/20191107220314_add_index_to_projects_on_marked_for_deletion.rb

+# frozen_string_literal: true
+
+class AddIndexToProjectsOnMarkedForDeletion < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def change
+    add_concurrent_index :projects, :marked_for_deletion_at, where: 'marked_for_deletion_at IS NOT NULL'
+  end
+end

db/schema.rb

@@ -3128,6 +3128,7 @@ ActiveRecord::Schema.define(version: 2019_11_11_115431) do
     t.index ["last_repository_check_at"], name: "index_projects_on_last_repository_check_at", where: "(last_repository_check_at IS NOT NULL)"
     t.index ["last_repository_check_failed"], name: "index_projects_on_last_repository_check_failed"
     t.index ["last_repository_updated_at"], name: "index_projects_on_last_repository_updated_at"
+    t.index ["marked_for_deletion_at"], name: "index_projects_on_marked_for_deletion_at", where: "(marked_for_deletion_at IS NOT NULL)"
     t.index ["marked_for_deletion_by_user_id"], name: "index_projects_on_marked_for_deletion_by_user_id", where: "(marked_for_deletion_by_user_id IS NOT NULL)"
     t.index ["mirror_last_successful_update_at"], name: "index_projects_on_mirror_last_successful_update_at"
     t.index ["mirror_user_id"], name: "index_projects_on_mirror_user_id"