PERF: index for deleted posts in a topic to speed up has_deleted detection

db/migrate/20160719002225_add_deleted_post_index_to_posts.rb

@@ -0,0 +1,5 @@
+class AddDeletedPostIndexToPosts < ActiveRecord::Migration
+  def change
+    add_index :posts, [:topic_id, :post_number], where: 'deleted_at IS NOT NULL', name: 'idx_posts_deleted_posts'
+  end
+end