Add index to resource milestone events

This adds a conditional index to improve query performance in burnup
charts

changelogs/unreleased/milestone-burnup-charts-backend.yml

+---
+title: Add index to resource_milestone_events for add actions
+merge_request: 37636
+author:
+type: other

db/migrate/20200729181641_add_index_to_resource_milestone_events_add_events.rb

+# frozen_string_literal: true
+
+class AddIndexToResourceMilestoneEventsAddEvents < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'index_resource_milestone_events_on_milestone_id_and_add_action'
+  ADD_ACTION = '1'
+
+  def up
+    # Index add milestone events
+    add_concurrent_index :resource_milestone_events, :milestone_id, where: "action = #{ADD_ACTION}", name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index :resource_milestone_events, :milestone_id, name: INDEX_NAME
+  end
+end

db/schema_migrations/20200729181641

+5dc4cbfc6d7e79e5909e5250f382bc3c9fa4246b8f2aed81404899aee4eef81b
\ No newline at end of file

db/structure.sql

@@ -20465,6 +20465,8 @@ CREATE INDEX index_resource_milestone_events_on_merge_request_id ON public.resou
 
 CREATE INDEX index_resource_milestone_events_on_milestone_id ON public.resource_milestone_events USING btree (milestone_id);
 
+CREATE INDEX index_resource_milestone_events_on_milestone_id_and_add_action ON public.resource_milestone_events USING btree (milestone_id) WHERE (action = 1);
+
 CREATE INDEX index_resource_milestone_events_on_user_id ON public.resource_milestone_events USING btree (user_id);
 
 CREATE INDEX index_resource_state_events_on_epic_id ON public.resource_state_events USING btree (epic_id);