RethinkDB might be another database to look at if this is functionality which you are interested in. My only gripe with doing this sort of thing is that if you offer a public API, it becomes separate to what you are using for the application internally. There are also some GraphQL databases like Prisma that offer this sort of functionality.
Make this good enough and you could very well cut into a huge chunk of Firebase's business model.
I'd seriously love to have query subscriptions, especially if the library is robust enough for use on mobile.
Ine large differentiator of Firebase's DB offerings is the subscription functionality, mobile and web, but using that means buying into NoSQL. A proper subscribable SQL database would be amazing.
Of course the conflict resolution on writes, and the local caching is another key benefit Firebase offers. Not sure how that could be done with a real relational DB!
Mainly because I want to keep my team small so it’s easier if we all commit to Postgres. But We also hard a hard time with Firebase’s filtering on sub collections (for their new Firestore)
