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From "Adam Kramer (JIRA)" <j...@apache.org>
Subject [jira] Created: (HIVE-836) Add syntax to force a new mapreduce job / transform subquery in mapper
Date Wed, 16 Sep 2009 17:35:57 GMT
Add syntax to force a new mapreduce job / transform subquery in mapper
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                 Key: HIVE-836
                 URL: https://issues.apache.org/jira/browse/HIVE-836
             Project: Hadoop Hive
          Issue Type: Wish
            Reporter: Adam Kramer


Hive currently does a lot of awesome work to figure out when my transformers should be used
in the mapper and when they should be used in the reducer. However, sometimes I have a different
plan.

For example, consider this:

SELECT TRANSFORM(a.val1, a.val2)
USING './niftyscript'
AS part1, part2, part3
FROM (
    SELECT b.val AS val1, c.val AS val2
    FROM tblb b JOIN tblc c on (b.key=c.key)
) a

...in this syntax b and c will be joined (in the reducer, of course), and then the rows that
pass the join clause will be passed to niftyscript _in the reducer._ However, when niftyscript
is high-computation and there is a lot of data coming out of the join but very few reducers,
there's a huge hold-up. It would be awesome if I could somehow force a new mapreduce step
after the subquery, so that ./niftyscript is run in the mappers rather than the prior step's
reducers.

Current workaround is to dump everything to a temporary table and then start over, but that
is not an easy to scale--the subquery structure effectively (and easily) "locks" the mid-points
so no other job can touch the table.

SUGGESTED FIX: Either cause MAP and REDUCE to force map/reduce steps (c.f. https://issues.apache.org/jira/browse/HIVE-835
), or add a query element to specify that "the job ends here." For example, in the above query,
FROM a SELF-CONTAINED or PRECOMPUTE a or START JOB AFTER a or something like that.


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