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Its a db company that offers an open source database and cloud managed services.

The database is OLAP where Postgres is an OLTP database. Essentially it very fast at complex queries, and is targeted at analytics workloads.

Postgres has been used as the basis for several OLAP systems. These guys are probably using a modified Greenplum.

As far as I am aware it is not a derivative of another database.

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