reduce the number map-reduce jobs for union all
-----------------------------------------------
Key: HIVE-2566
URL: https://issues.apache.org/jira/browse/HIVE-2566
Project: Hive
Issue Type: Improvement
Reporter: Namit Jain
Assignee: Namit Jain
A query like:
select s.key, s.value from (
select key, value from src2 where key < 10
union all
select key, value from src3 where key < 10
union all
select key, value from src4 where key < 10
union all
select key, count(1) as value from src5 group by key
)s;
should run the last sub-query
'select key, count(1) as value from src5 group by key'
as a map-reduce job.
And then the union should be a map-only job reading from the first 3 map-only
subqueries
and the output of the last map-reduce job.
The current plan is very inefficient.
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