Add/Verify Greenplum Support

[Xangis]

The Greenplum database, at https://github.com/greenplum-db/gpdb has recently been open-sourced.

It is based on PostgreSQL and is commonly used in many of the same use cases as Redshift (columnar/sharded Postgres).

The PostgreSQL module may work for Greenplum without modification. Or it might not. It would be great to have official support for it. I'll try redash against a GPDB to see if it works and should be able to report back in a few days.

[arikfr]

👍 supporting Greenplum will be great. I don't use it myself, so can't help much with the testing. But if you have any questions, let me know -- I'll be happy to assist.

[Xangis]

I've done a bit of Redash testing against a small Greenplum cluster and it works just fine with the PostgreSQL driver (and doesn't require any special settings). That makes sense, since tools like pgAdmin work with Greenplum.

[arikfr]

Closed with #688.