When do I need map reduce for database queries?

Question

In CouchDB you always have to use map reduce to query results.

In MongoDB you can their query methods for retrieving data, but they also let you do map-reduce.

I wonder, when do I actually need map-reduce?

Are those query methods different from map-reduce or are they just wrappers for map-reduce functions?

Answer 1

MapReduce is needed for aggregations in MongoDB. The normal queries follow a very different (and much faster) code path and they should always be used for real-time operations. MapReduce is definitely not intended for real-time, it's more for batch jobs.

Technically, you could write all your queries using MapReduce, but that would be both painful and slow.