-- union case: both subqueries are a map-only jobs, same input, followed by filesink

explain 
  select unionsrc.key, unionsrc.value FROM (select s1.key as key, s1.value as value from src s1 UNION  ALL  
                                            select s2.key as key, s2.value as value from src s2) unionsrc;

select unionsrc.key, unionsrc.value FROM (select s1.key as key, s1.value as value from src s1 UNION  ALL  
                                          select s2.key as key, s2.value as value from src s2) unionsrc;