Hello,
There's some ambiguity whether a union array's "types" buffer stores physical child ids, or logical type codes. Some of our C++ tests assume the former: https://github.com/apache/arrow/blob/master/cpp/src/arrow/array_union_test.cc#L107-L123 Some of our C++ tests assume the latter: https://github.com/apache/arrow/blob/master/cpp/src/arrow/array_union_test.cc#L311-L326 https://github.com/apache/arrow/blob/master/cpp/src/arrow/ipc/json_simple_test.cc#L943-L955 Critically, no validation of union data is currently implemented in C++ (ARROW-6157). I can't parse the Java source code. Regards Antoine.
