MongoDB: Terrible MapReduce Performance

Question

I have a long history with relational databases, but I\'m new to MongoDB and MapReduce, so I\'m almost positive I must be doing something wrong. I\'ll jump right into the qu

Answer 1

You are not doing anything wrong. (Besides sorting on the wrong value as you already noticed in your comments.)

MongoDB map/reduce performance just isn't that great. This is a known issue; see for example http://jira.mongodb.org/browse/SERVER-1197 where a naive approach is ~350x faster than M/R.

One advantage though is that you can specify a permanent output collection name with the out argument of the mapReduce call. Once the M/R is completed the temporary collection will be renamed to the permanent name atomically. That way you can schedule your statistics updates and query the M/R output collection real-time.