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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.