Greenplum is the worlds first open-source massively parallel processing database based on PostgreSQL.It provides powerful and rapid analytics on petabyte scale data volumes
PostgreSQL is an open-source, object-relational database management system (ORDBMS) available for all major platforms including Linux, UNIX, Windows and OS X
"According to the postgresql which i gather greenplum is based on documentation for sequence manipulation functions it should return the value most recently returned by nextval in the current session."
from question "Get last inserted row ID with Psycopg2 and a Greenplum database"
"Greenplum and other similar solutions should work a bit better than postgresql depending on your data sets and use cases"
from question "Postgresql for OLAP"