From a technical point of view Materialize seems absolutely fantastic - everything that the Firebase databases are trying to be, except with full SQL support (well, minimal SQL support - there's no RETURNING, ON CONFLICT or json - but probably good enough, we at least get JOINs and CTEs).

But unfortunately they only seem to be interested in enterprise customers: Not only is it not open source, but there's no open sign up and pricing for compute nodes isn't publicly available!