add index on post_versions.updated_at

db/migrate/20130221032344_add_index_updated_at_on_post_versions.rb

@@ -0,0 +1,9 @@
+class AddIndexUpdatedAtOnPostVersions < ActiveRecord::Migration
+  def up
+    add_index :post_versions, :updated_at
+  end
+
+  def down
+    remove_index :post_versions, :updated_at
+  end
+end