PERF: reverse index on topic users to speed up join

db/migrate/20140910130155_create_topic_user_index.rb

@@ -0,0 +1,6 @@
+class CreateTopicUserIndex < ActiveRecord::Migration
+  def change
+    # seems to be the most effective for joining into topics
+    add_index :topic_users, [:user_id, :topic_id], unique: true
+  end
+end