PERF: add missing index on post actions

db/migrate/20150513094042_add_index_on_post_actions.rb

@@ -0,0 +1,5 @@
+class AddIndexOnPostActions < ActiveRecord::Migration
+  def change
+    add_index :post_actions, [:user_id, :post_action_type_id], where: 'deleted_at IS NULL'
+  end
+end