set hive.map.aggr = true; -- SORT_BEFORE_DIFF -- union case: 1 subquery is a map-reduce job, different inputs for sub-queries, followed by reducesink explain select unionsrc.key, count(1) FROM (select s2.key as key, s2.value as value from src1 s2 UNION ALL select 'tst1' as key, cast(count(1) as string) as value from src s1) unionsrc group by unionsrc.key; select unionsrc.key, count(1) FROM (select s2.key as key, s2.value as value from src1 s2 UNION ALL select 'tst1' as key, cast(count(1) as string) as value from src s1) unionsrc group by unionsrc.key;