excerpts from MongoDB Definitive Guide from O’Reilly:
The price of using MapReduce is speed:
group is not particularly speedy, but
MapReduce is slower and is not
supposed to be used in “real time.”
You run MapReduce as a background
job, it creates a collection of
results, and then you can query that
collection in real time.
options for map/reduce:
"keeptemp" : boolean
If the temporary result collection should be saved when the connection is closed.
"output" : string
Name for the output collection. Setting this option implies keeptemp : true.