when union kudu table and hdfs table, union passthrough does not take effect

IMPALA-3586 already supports union passthrough, and brings great performance improvements in union， but there is still some problems when union between hdfs table and kudu table ，several points cause the problem:

1. in kudu scanner node output TupleDescriptor, string slot is 16B,while in hdfs scanner node output TupleDescriptor, string slot is 12B,cause tuple memory layout mismatch
2. in kudu scanner node output TupleDescriptor, string slot is 16B, while in Union output TupleDescriptor, string slot is 12B,cause tuple memory layout mismatch
3. in Kudu Scannode, row key slot is not null, while in hdfs node, not null slot can't get from the metadata, cause tuple memory layout mismatch

I hive resolved the 1st and 2nd points, how should I do with the 3rd point?