Often we will sort the BSON elements of a $in list early in query processing. Sorting BSON elements is usually fairly slow, and for short running query, this takes a significant amount of the runtime.

Sometimes, this $in list gets converted to KeyStrings, which are used for index bounds. KeyStrings are very easy/fast to compare, so we should try to delay the sorting of the $in list elements until after the KeyStrings are generated.

This will not be possible in all cases, but should be a minor perf win for $in queries that get answered with ixscan/fetch plans. For example, a query that fetches a bunch of documents via a $in on _id.