Parallel execution is insufficient for some use cases. It leads to deadlock which the application needs to handle. For a legacy application without that handling, the choice of serial execution avoids deadlock. This is exactly the problem I had using HA-JDBC and MariaDB+Galera with parallel execution, but HA-JDBC offered serial write execution as a fallback. I was able to migrate to HA-JDBC right away using serial execution, and I am working towards adding application handling so I can take advantage of HA-JDBC parallel execution for the same write amplification being touted for PosgresXC. If PostgresXC offered the choice of parallel or serial execution, I would love to switch from HA-JDBC.
Postgres-xc works wonderful.
Not yet mature (as of Jan 2013) but very promising.
Very good project, at last an real high availability sql server begin to exist.
Most promising scale-out PostgreSQL clusetr.