| How well do foreign data wrappers work with Postgres's query planner?

You can provide table size estimation to Postgres's query planner in wrappers using `get_rel_size()` function, here is doc:

https://docs.rs/supabase-wrappers/latest/supabase_wrappers/i...

| Also, is the Firebase connector for Realtime Database or Firestore?

It is for Firestore, Realtime Database is not supported yet.