//////////////////////////////////////////////////////////
// compare KEY, values from DataJoinKey#KEY_FIELDNAME
// KEY represents the actual key user specified
///////////////////////////////////////////////////////////
byte type1 = d1.readByte();
byte type2 = d2.readByte();
_COLUMN_COMPARATOR.setType(type1, type2);
// writable, check if they are Tuple or NullWritable
if( type1==Tuple.NULL_WRITABLE_TYPE && type2==Tuple.NULL_WRITABLE_TYPE )