Implementing aggregation at query time is a temporary solution. For systems like this aggregation should be done on insert time - many hugely popular databases do not provide much more than a basic get operation for this reason

You might be right but Mongo supports them and Microsoft says that we can start using DocumentDb without changing any line of code which is not true.

Yeah I don't get why they're marketing to Mongo users, weird choice. This is a DB for companies trying to moved to a query-less architecture - something no one should be doing with mongo

I never thought about it that way but makes total sense. Clearly you 'get' it!