Speaking of Tandem Databases, HP had released the SQL engine behind SQL/MX[0] as open source (Trafodion) running in front of Hadoop to the Apache Software Foundation but it appears they have shutdown the project[1].

[0]: https://thenewstack.io/sql-hadoop-database-trafodion-bridges...

[1]: https://attic.apache.org/projects/trafodion.html