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Query to find the oldest records in the catalog. Nothing should be more than a day older than the control_file_record_keep_time parameter.

set lines 1000
select db_name
,      input_type
,      min(start_time)
from   rc_rman_backup_job_details
where  1=1
group  by db_name
,      input_type
order  by 3
/

Using straight data pump to export a sample does not take referential constraints into account.
Using this method will fix that oversight.
http://www.dba-oracle.com/t_data_pump_export_rows_subset.htm