-- union :map-reduce sub-queries followed by join

explain 
SELECT unionsrc1.key, unionsrc1.value, unionsrc2.key, unionsrc2.value
FROM (select 'tst1' as key, cast(count(1) as string) as value from src s1
                         UNION  ALL  
      select s2.key as key, s2.value as value from src s2 where s2.key < 10) unionsrc1 
JOIN 
     (select 'tst1' as key, cast(count(1) as string) as value from src s3
                         UNION  ALL  
      select s4.key as key, s4.value as value from src s4 where s4.key < 10) unionsrc2
ON (unionsrc1.key = unionsrc2.key);

SELECT unionsrc1.key, unionsrc1.value, unionsrc2.key, unionsrc2.value
FROM (select 'tst1' as key, cast(count(1) as string) as value from src s1
                         UNION  ALL  
      select s2.key as key, s2.value as value from src s2 where s2.key < 10) unionsrc1 
JOIN 
     (select 'tst1' as key, cast(count(1) as string) as value from src s3
                         UNION  ALL  
      select s4.key as key, s4.value as value from src s4 where s4.key < 10) unionsrc2
ON (unionsrc1.key = unionsrc2.key);