Optimize query for usage ping

Query by adding an index and using batch counting

changelogs/unreleased/208923-enable-batch-counting-for-some-individual-queries-2.yml

+---
+title: Optimize template_repositories query by using batch counting
+merge_request: 27352
+author:
+type: performance

db/migrate/20200316162648_add_index_on_namespace_id_and_id_to_projects.rb

+# frozen_string_literal: true
+
+class AddIndexOnNamespaceIdAndIdToProjects < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :projects, [:namespace_id, :id]
+    remove_concurrent_index :projects, :namespace_id
+  end
+
+  def down
+    add_concurrent_index :projects, :namespace_id
+    remove_concurrent_index :projects, [:namespace_id, :id]
+  end
+end

db/structure.sql

@@ -9657,7 +9657,7 @@ CREATE INDEX index_projects_on_name_and_id ON public.projects USING btree (name,
 
 CREATE INDEX index_projects_on_name_trigram ON public.projects USING gin (name public.gin_trgm_ops);
 
-CREATE INDEX index_projects_on_namespace_id ON public.projects USING btree (namespace_id);
+CREATE INDEX index_projects_on_namespace_id_and_id ON public.projects USING btree (namespace_id, id);
 
 CREATE INDEX index_projects_on_path_and_id ON public.projects USING btree (path, id);
 
@@ -12702,6 +12702,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20200313101649'),
 ('20200313123934'),
 ('20200316111759'),
+('20200316162648'),
 ('20200316173312'),
 ('20200317142110'),
 ('20200318152134'),

ee/lib/ee/gitlab/usage_data.rb

@@ -157,7 +157,7 @@ module EE
                                          projects_with_packages: count(::Packages::Package.select('distinct project_id'), batch: false),
                                          projects_with_prometheus_alerts: distinct_count(PrometheusAlert, :project_id),
                                          projects_with_tracing_enabled: count(ProjectTracingSetting),
-                                         template_repositories: count(::Project.with_repos_templates, batch: false) + count(::Project.with_groups_level_repos_templates, batch: false)
+                                         template_repositories: count(::Project.with_repos_templates) + count(::Project.with_groups_level_repos_templates)
                                        },
                                        service_desk_counts,
                                        security_products_usage,