Join-reordering is intended to be applied to top-level aggregation pipelines. This ticket is to confirm that an in-eligible top-level pipeline containing a $lookup with a subpipeline that itself is eligible for join re-ordering is not permitted to use join re-ordering.

It would not be ideal if the sub-pipeline used join re-ordering. That’s because the top-level $lookup would execute in document source land, where its subpipeline could get re-optimized potentially many times. We don’t want to incur the cost of using join optimization repeatedly.