Impala's sorting code currently only allows for full sorts, but it could be extended to support partial sorts.
This would be useful in situations where the sorting is being done for performance rather than correctness. For example, a recent change (
IMPALA-3742) sorts rows for an INSERT before sending them to Kudu. Doing only a partial sort could speed this up and reduce the memory required while still retaining the primary benefit of reducing the load on Kudu.