There is no relevant knowledge that Postgres has that RethinkDB lacks that lets it evaluate the query more efficiently (besides maybe a row layout with fixed offsets so that it doesn't haven't parse documents, but that's not relevant to the reported problem). A generic map reduce certainly would have more overhead, obviously, but not running-out-of-memory overhead reported above, just the overhead of merging big documents.
The reason you run queries in "the core" of a database is because copying all the data outside the database and doing computations there would be far worse.
The reason you run queries in "the core" of a database is because copying all the data outside the database and doing computations there would be far worse.