A large number of nScannedObjects occurs when a sharded $in query contains both a limit() and sort() clause.

For example, we have a checkins collection which is sharded by

and contains the index

. A db.checkins.find({uid: {$in: [400438]}}).limit(2).sort({_id: -1}) will scan just the two documents returned. db.checkins.find({uid: {$in: [400438, 32]}}).limit(2).sort({_id: -1}) will scan thousands of documents.

The attachment walks through the example with an interactive console.