Add index to improve query performance

For Environment.deployed_to_cluster

changelogs/unreleased/cluster_deployments.yml

+---
+title: Add index to improve group cluster deployments query performance
+merge_request: 31988
+author:
+type: other

db/migrate/20190819131155_add_cluster_status_index_to_deployments.rb

+# frozen_string_literal: true
+
+class AddClusterStatusIndexToDeployments < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :deployments, [:cluster_id, :status]
+  end
+
+  def down
+    remove_concurrent_index :deployments, [:cluster_id, :status]
+  end
+end

db/schema.rb

@@ -1146,6 +1146,7 @@ ActiveRecord::Schema.define(version: 2019_08_28_083843) do
     t.integer "status", limit: 2, null: false
     t.datetime_with_timezone "finished_at"
     t.integer "cluster_id"
+    t.index ["cluster_id", "status"], name: "index_deployments_on_cluster_id_and_status"
     t.index ["cluster_id"], name: "index_deployments_on_cluster_id"
     t.index ["created_at"], name: "index_deployments_on_created_at"
     t.index ["deployable_type", "deployable_id"], name: "index_deployments_on_deployable_type_and_deployable_id"