PERF: speed up counting of unread notifications

db/migrate/20150514023016_add_unread_notifications_index.rb

@@ -0,0 +1,5 @@
+class AddUnreadNotificationsIndex < ActiveRecord::Migration
+  def change
+    add_index :notifications, [:user_id, :notification_type], where: 'not read', name: 'idx_notifications_speedup_unread_count'
+  end
+end