The unpack bucket stage first checks if any rewrites are possible , if so, does push downs, and only after this tries to optimize the end of the pipeline. This results in sub-optimal scenarios. For example, for `db.coll.aggregate([\{$sort: {"m": 1}}, \{$skip: 1}, \{$limit: 1}])`, `$sort` gets pushed down before `_internalUnpackBucket` instead of topKSort. However, in cases such as

db.coll.aggregate([
    {
        "$match": {
            "$nor": [
                {"t": {"$in": []}},
            ]
        }
    },
    {"$sort": {"m": 1}},
    {"$skip": 1},
    {"$limit": 1}
]
)

topKSort is pushed down because the presence of `$match` stage between  `_internalUnpackBucket` and `$sort` results in the optimization of the end of the pipeline.