If we look at the dates listed in the Wiki, Greenplum is listed as starting in 2005. They’re not kidding, and unfortunately it seems Pivotal executed a “fork it and forget it” maneuver. The documentation admits Greenplum is based on Postgres 8.2, with elements of functionality from 8.3.
Like Amazon’s Redshift, this immediately disqualifies Greenplum from consideration for anyone using a newer version. Our own databases are on 9.4 pending an upgrade plan; there’s no way we could justify such a massive downgrade, even for horizontal scaling improvements. EnterpriseDB had a similar problem when they started selling their version of 8.3; they were far behind for years before they managed to reduce their version lag by only a few months. Greenplum never even bothered.
This may be an amazing product, but we can’t use it to replace existing Postgres 9.4 databases that need scaling. Will Greenplum catch up now that it’s been open-sourced? I can’t say. It would definitely be cool, but I’m not holding my breath. Incidentally, this is one of the reasons all of those projects on the Wiki have definitive end dates. Keeping up with Postgres after forking is extremely difficult if you don’t merge your enhancements back into core. It’s all too easy to fall hopelessly behind and become nothing but an academic concern.