Namit Jain created HIVE-3276:
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Summary: optimize union sub-queries
Key: HIVE-3276
URL: https://issues.apache.org/jira/browse/HIVE-3276
Project: Hive
Issue Type: Bug
Reporter: Namit Jain
Assignee: Nadeem Moidu
It might be a good idea to optimize simple union queries containing map-reduce
jobs in at least one of the sub-qeuries.
For eg:
a query like:
insert overwrite table T1 partition P1
select * from
(
subq1
union all
subq2
) u;
today creates 3 map-reduce jobs, one for subq1, another for subq2 and
the final one for the union.
It might be a good idea to optimize this. Instead of creating the union
task, it might be simpler to create a move task (or something like a move
task), where the outputs of the two sub-queries will be moved to the final
directory. This can easily extend to more than 2 sub-queries in the union.
This is only useful if there is a select * followed by filesink after the
union. This can be independently useful, and also be used to optimize the
skewed joins https://cwiki.apache.org/Hive/skewed-join-optimization.html
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