Create /data/a1/pkey=1 directory with some data in it.
Create /data/a2/pkey=1 directory with some data in it.
create external table a1 (value string) partitioned by (pkey int) location '/data/a1';
create external table a2 (value string) partitioned by (pkey int) location '/data/a2';
alter table a2 add partition (pkey=1);
alter table a1 exchange partition (pkey=1) with table a2;
select * from a1 should now fail.
pkey=1 is not a partition of a1 but the folder exists. We should give an error message for that.