In which cases am I better using DBs like Couch or Mongo as opposed to traditional RDBMS

Question

In which cases am I better using DBs like Couch or Mongo as opposed to traditional RDBMS.
What is the problem they are suppose to solve (or solve more efficiently) then RDBMSs?

Answer 1

Relational databases traditionally store data in rows within tables. This isn't a natural fit for documents, which have a hierarchical structure.

Using Couch/Mongo (or similar) means you don't have to decompose hierarchical objects into a set of tables/rows. This decomposition can be painful to implement, and extensibility can be a problem when the shape of the object changes. Object-Relational mappers (ORMs) aim to solve this problem automatically.

I can't comment on the efficiency, I'm afraid. I suspect that's strongly implementation-specific (in terms of how you decompose your objects, what you're querying on etc.)