Optimize ci builds AMAU queries

Adds an index to ensure computation for the queries

changelogs/unreleased/208884-optimize-ci_builds-counters-in-usage-data.yml

+---
+title: Optimize ci builds counters in usage data
+merge_request: 27770
+author:
+type: performance

db/migrate/20200323122201_add_index_on_user_and_created_at_to_ci_builds.rb

+# frozen_string_literal: true
+
+class AddIndexOnUserAndCreatedAtToCiBuilds < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'index_ci_builds_on_user_id_and_created_at_and_type_eq_ci_build'
+
+  def up
+    add_concurrent_index :ci_builds, [:user_id, :created_at], where: "type = 'Ci::Build'", name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index :ci_builds, INDEX_NAME
+  end
+end

db/structure.sql

@@ -8603,6 +8603,8 @@ CREATE INDEX index_ci_builds_on_upstream_pipeline_id ON public.ci_builds USING b
 
 CREATE INDEX index_ci_builds_on_user_id ON public.ci_builds USING btree (user_id);
 
+CREATE INDEX index_ci_builds_on_user_id_and_created_at_and_type_eq_ci_build ON public.ci_builds USING btree (user_id, created_at) WHERE ((type)::text = 'Ci::Build'::text);
+
 CREATE INDEX index_ci_builds_project_id_and_status_for_live_jobs_partial2 ON public.ci_builds USING btree (project_id, status) WHERE (((type)::text = 'Ci::Build'::text) AND ((status)::text = ANY (ARRAY[('running'::character varying)::text, ('pending'::character varying)::text, ('created'::character varying)::text])));
 
 CREATE UNIQUE INDEX index_ci_builds_runner_session_on_build_id ON public.ci_builds_runner_session USING btree (build_id);
@@ -12746,5 +12748,6 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20200318165448'),
 ('20200318175008'),
 ('20200319203901'),
-('20200323075043');
+('20200323075043'),
+('20200323122201');