I think ToroDB is super cool, and I wish your project the best of luck! You can't go wrong building something on PostgreSQL. :)

That said, PostgreSQL FDW is NOT a great option for MongoDB analytics. Not only is the data model so different that you lose the ability to answer many types of questions, but Multicorn supports only basic pushdown (conjunctions of simple relational operators on original columns).

What this means is that analytics via PostgreSQL via FDW via Multicorn via MongoDB suffers from (a) very poor expressive power, and (b) ridiculously slow performance, since nearly any type of query will require at least one full table scan on all the source tables (in some cases, especially with arrays, many more full table scans may be required for a single query!).

Better off just using ToroDB. Am I right? :)