One of the tables named in the ALTER TABLE EXCHANGE SUBPARTITION command has a UNIQUE constraint for which no matching (vis-a-vis key columns) constraint is defined on the other table or a matching constraint is defined on the other table, but it differs from that defined on the first table vis-a-vis being enabled and/or validated.
Ensure that all the rows in the segment qualify for the subpartition. Perform the alter table operation with the NO CHECKING option. Run ANALYZE table VALIDATE on that subpartition to find out the invalid rows and delete them.
Try the following links for this error:
http://docs.oracle.com/cd/E16655_01/server.121/e26079/e12700.htm
https://community.oracle.com/message/2503397
https://community.oracle.com/message/2503582
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