Add index for optimizing service desk counter

- needed for usage ping counts

changelogs/unreleased/208887-optimize-project-counters-service-desk.yml

+---
+title: Optimize Project related count service desk enabled
+merge_request: 27115
+author:
+type: performance

db/migrate/20200312163407_add_index_on_id_and_service_desk_enabled_to_projects.rb

+# frozen_string_literal: true
+
+class AddIndexOnIdAndServiceDeskEnabledToProjects < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_projects_on_id_service_desk_enabled'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :projects, :id, where: 'service_desk_enabled = true', name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :projects, INDEX_NAME
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2020_03_11_165635) do
+ActiveRecord::Schema.define(version: 2020_03_12_163407) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -3485,6 +3485,7 @@ ActiveRecord::Schema.define(version: 2020_03_11_165635) do
     t.index ["id", "repository_storage", "last_repository_updated_at"], name: "idx_projects_on_repository_storage_last_repository_updated_at"
     t.index ["id"], name: "index_on_id_partial_with_legacy_storage", where: "((storage_version < 2) OR (storage_version IS NULL))"
     t.index ["id"], name: "index_projects_on_id_partial_for_visibility", unique: true, where: "(visibility_level = ANY (ARRAY[10, 20]))"
+    t.index ["id"], name: "index_projects_on_id_service_desk_enabled", where: "(service_desk_enabled = true)"
     t.index ["id"], name: "index_projects_on_mirror_and_mirror_trigger_builds_both_true", where: "((mirror IS TRUE) AND (mirror_trigger_builds IS TRUE))"
     t.index ["last_activity_at", "id"], name: "index_projects_api_last_activity_at_id_desc", order: { id: :desc }
     t.index ["last_activity_at", "id"], name: "index_projects_api_vis20_last_activity_at", where: "(visibility_level = 20)"