docs: public static Writable min(JavaRDD<List<Writable>> allData, String columnName, Schema schema){
          int columnIdx = schema.getIndexOfColumn(columnName);
          JavaRDD<Writable> col = allData.map(new SelectColumnFunction(columnIdx));
          return col.min(Comparators.forType(schema.getType(columnName).getWritableType()));
      }