Ben Foster looks at MongoDB’s Map-Reduce and aggregation framework and then compares them with RavenDB’s Map-Reduce:
I thought it would be interesting to do a practical comparison of Map-Reduce
in both MongoDB and RavenDB.
There are more differences than similarities — I’m not referring to the API differences, but to fundamental differences to the ways they operate.
✚ RavenDB’s author has a follow up post in which he underlines another major difference: RavenDB’s Map-Reduce operates as an index, while MongoDB’s Map-Reduce is an online operation.
Original title and link: A practical comparison of Map-Reduce in MongoDB and RavenDB