Pivotal Greenplum Database is a MPP (massively parallel processing) relational database built on open source PostgreSQL.
It's a very capable open-source data warehouse, able to work with petabytes of data. Greenplum is geared towards big data analytics. Work is performed by segments nodes, who run on one or more segments - and each segment is a modified PostgreSQL database instance.
Greenplum can act as a standard transactional database, as it supports ACID. However, it's optimized for analytical workloads, not for OLTP.
As Greenplum is derived from PostgreSQL and is compatible in connection mechanisms with it, all our applications that work with PostgreSQL will work perfectly with Greenplum database.