docs: public static Map<String,Set<Writable>> getUnique(List<String> columnNames, Schema schema, RecordReader data){
          Map<String,Set<Writable>> m = new HashMap<>();
          for(String s : columnNames){
              m.put(s, new HashSet<>());
          }
          while(data.hasNext()){
              List<Writable> next = data.next();
              for(String s : columnNames){
                  int idx = schema.getIndexOfColumn(s);
                  m.get(s).add(next.get(idx));
              }
          }
          return m;
      }