Despite that, can't say for a fact that's the case for Relax since I haven't test both on it. Mongo is behaving really well though, our demo instance has been getting a pound lately (dozens of users at a time) and it's running smoothly even though our machine is not really powerful.
Having this said, not entirely against having different database layers supported in Relax. Since we're using GraphQL would be a matter of creating an abstraction when accessing the data on the queries and mutations resolves. Not on our priorities for now but we're always open for contributions :)
Saying PostgreSQL is a better fit in terms of performance is a bit controversial in my opinion, there are quite a lot of tests between the two and most say it is the exact opposite https://blog.michaelckennedy.net/2010/04/29/mongodb-vs-sql-s...
Despite that, can't say for a fact that's the case for Relax since I haven't test both on it. Mongo is behaving really well though, our demo instance has been getting a pound lately (dozens of users at a time) and it's running smoothly even though our machine is not really powerful.
Also in terms of scaling, Mongo also has a great solution for horizontal scaling https://docs.mongodb.org/manual/sharding/
Having this said, not entirely against having different database layers supported in Relax. Since we're using GraphQL would be a matter of creating an abstraction when accessing the data on the queries and mutations resolves. Not on our priorities for now but we're always open for contributions :)