If we want to sort our results, there are two options:

• If we are using the shard key in our sort criteria, then mongos can determine the order in which it has to query the shard or shards. This results in an efficient and, again, targeted operation.
• If we are not using the shard key in our sort criteria, then as with a query without sort, it's going to be a fanout query. To sort the results when we are not using the shard key, the primary shard executes a distributed merge sort locally before passing on the sorted result set to mongos.

Limit on queries is enforced on each individual shard and then again at the mongos level as there may be results from multiple shards.

Skip, on the other hand, cannot be passed on to individual shards and will be applied by mongos after retrieving all the results locally.