While Hive has UNION ALL feature, it results in duplicate records and customers have to write another complex query or MapReduce job to Dedup the duplicate records. UNION feature is must have in Hive as it is in all SQL RDBMS as well as in Apache Impala.
Not sure how much effort will it take given the fact that UNION ALL is already available.
- is related to
HIVE-9039 Support Union Distinct