At the time of writing, the cluster distinct command sets up a BSONObjSet to deduplicate results across shards. In the general case this is necessary, but when targeting a single shard, this is unnecessary. As such, this ticket tracks the work to return the lone shard response directly in the case of targeting a single shard.

For context, we target a single shard when:

• The target namespace is unsharded (either untracked or unsplittable)
• The target namespace is sharded and all targeted chunks reside on a single shard