Add an index on message sender.

db/migrate/038_add_message_sender_index.rb

@@ -0,0 +1,9 @@
+class AddMessageSenderIndex < ActiveRecord::Migration
+  def self.up
+    add_index :messages, [:from_user_id], :name=> "messages_from_user_id_idx"
+  end
+
+  def self.down
+    drop_index :messages, :name=> "messages_from_user_id_idx"
+  end
+end