Why are temporal extensions in PostgreSQL important? Quite simply, managing time data is one of the most common requirements, and current general-purpose database systems don’t provide us with the basic tools to do it. Every general-purpose DBMS falls short both in terms of usability and performance when trying to manage temporal data.
What is already done?
The relational model is just a way to represent reality. It happens to have some very useful properties, such as closure over many useful operations — but it’s a purely logical model of reality. You can implement relational operations using hash joins, MapReduce, or pen and paper.
So, right away, it’s meaningless to talk about the scalability of the relational model. Given a particular question, it might be difficult to break it down into bite-sized pieces and distribute it to N worker nodes. But going with MapReduce doesn’t solve that scalability problem — it just means that you will have a hard time defining a useful map or reduce operation, or you will have to change the question into something easier to answer.