HI 

Working with a Client we were exploring ways to mitigate load issues on the mongod hosts on a Sharded cluster. 

The particular client is doing multiple $in statements in each query but have large lists for up to 1000 entries per $in part.  Based on testing on both Our and the client side we were able to see significant advantages of trying to target each $in's entries to a single shard. Thus avoiding scatter gather queries foe each fo the $in queries.

The optimisation could be both on the mongos or on the driver side, and on the dirver and could make it invisible to the client.