PostgreSQL has a feature called Foreign Data Wrappers. It's certainly not what th OP imagins, but still pretty cool to query and join external data sources.
I was not aware of this feature. It is REALLY cool...even if I’m struggling to think of a situation where I would prefer it over doing the “joins” in application code, at least for the drivers I looked at when I searched this space. If “smart” drivers already existed for making sense of schemas across platforms (including performance characteristics that Postgres would leverage) then I would drop everything and go all in on this.
