I was looking for a open source parallel database solution. It seems Postgres-XC is a viable solution. However, I don't need the replication. In this case, is there any better solution towards read optimized parallel architecture ?
There is pl/proxy, which provides a basic federation capability, but it's not an engine optimised for this sort of work. Greenplum is a commercial product that does have a shared nothing engine that might do what you want. However, it's not cheap - less expensive than Oracle but more expensive than SQL Server.
There are no pure open-source engines of that type for PostgreSQL. However, you might get what you want from PL/Proxy.