datavirtue parent
Postgres has been used as the basis for several OLAP systems. These guys are probably using a modified Greenplum.
I got to see Citus at Microsoft fail to close against ClickHouse for an internal project
ClickHouse spun out of Yandex & is open source, https://github.com/ClickHouse/clickhouse
Disclosure: I started at Citus & ended up at ClickHouse
As far as I am aware it is not a derivative of another database.