With the pipeline of [$lookup, $unwind, $sort, $limit], the current implementation will not push $sort before $lookup since $unwind will block $limit from being pushed before $lookup and then combined with $sort. Although letting $sort take advantages of the indexes benefits more than absorbing $limit into $sort to run a top-k sort, there is no way in the current system to learn if there are indexes on the sorting fields.

Instead of depending on the presence of $unwind, the optimization should reorder $lookup and $sort depending on if an index-provided sort is allowed. The other condition of not sorting on fields introduced or modified by $lookup and $unwind remains the same.