other alternatives:

now:

* Pivotal Greenplum Database - is now Open Source : https://github.com/greenplum-db/gpdb

later:

* CitusDB 5.0 will be open source (2016Q1-Q2) :

( "An extension of PostgreSQL to be distributed/parallel query engine" )

https://twitter.com/frsyuki/status/667031289506062336

http://info.citusdata.com/CitusDB-5-Pre-Beta.html

* PostgreSQL 9.6 will have a "Parallel Sequential Scans" :

http://amitkapila16.blogspot.hu/2015/11/parallel-sequential-...