Optimize projects_service_active queries

by adding an index and converting the
grouping to individual batch queries

changelogs/unreleased/208923-enable-batch-counting-for-some-individual-queries-4.yml

+---
+title: Optimize projects_service_active queries performance in usage data
+merge_request: 27093
+author:
+type: performance

db/migrate/20200312125121_add_index_on_active_and_template_and_type_and_id_to_services.rb

+# frozen_string_literal: true
+
+class AddIndexOnActiveAndTemplateAndTypeAndIdToServices < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_services_on_type_and_id_and_template_when_active'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :services, [:type, :id, :template], where: 'active = TRUE', name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index :services, INDEX_NAME
+  end
+end

db/schema.rb

@@ -3964,6 +3964,7 @@ ActiveRecord::Schema.define(version: 2020_03_12_163407) do
     t.boolean "instance", default: false, null: false
     t.index ["project_id", "type"], name: "index_services_on_project_id_and_type"
     t.index ["template"], name: "index_services_on_template"
+    t.index ["type", "id", "template"], name: "index_services_on_type_and_id_and_template_when_active", where: "(active = true)"
     t.index ["type", "instance"], name: "index_services_on_type_and_instance", unique: true, where: "(instance IS TRUE)"
     t.index ["type", "template"], name: "index_services_on_type_and_template", unique: true, where: "(template IS TRUE)"
     t.index ["type"], name: "index_services_on_type"