With memchached you pay the performance price for interprocess communication to get to the data in your hashtable, even when the data are on the same machine. The RoR / PHP / Python models for web development have each request running in different process, so this is unavoidable for them. Java / C++ are more suitable for pg's architecture (obviously Arc / Lisp too).

Also when you use memchached as a db cache, be preprared to lose some transactional properies of the database storage.