code: public static Set<Writable> getUnique(String columnName, Schema schema, RecordReader data) {
          int colIdx = schema.getIndexOfColumn(columnName);
          Set<Writable> unique = new HashSet<>();
          while(data.hasNext()){
              List<Writable> next = data.next();
              unique.add(next.get(colIdx));
          }
          return unique;
      }
docs: Get a list of unique values from the specified columns.
  For sequence data, use {@link #getUniqueSequence(List, Schema, SequenceRecordReader)}
  
  @param columnName    Name of the column to get unique values from
  @param schema        Data schema
  @param data          Data to get unique values from
  @return              List of unique values