This currently happens automatically if the split-point is a $group, but not if it is a $sort, $skip, or $limit. If any of the mongos-only stages return EXHAUSTIVE from getDependencies(), but none of the shard stages do, then a $project should be added to the end of the shard stages.

Advantages:

• reduce mongod -> mongos network traffic
• allow mongod to reify only needed fields
• allow more documents to be output from a shard before hitting 16MB limit.