I'm going to be working on: https://issues.apache.org/jira/browse/IMPALA-5498, adding support for partial sorts to Impala.
I've written up a brief design doc outlining my thoughts, if anyone would like to look over it and make suggestions: https://docs.google.com/document/d/1g4eJqBaZa8taA1cSsP-TJ-Mp92u8MGdYuh6bf2R51hg/edit?usp=sharing
