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From "Namit Jain (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HIVE-3276) optimize union sub-queries
Date Thu, 19 Jul 2012 04:44:34 GMT
Namit Jain created HIVE-3276:
--------------------------------

             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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