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From "Namit Jain (JIRA)" <j...@apache.org>
Subject [jira] Created: (HIVE-1772) optimize join followed by a groupby
Date Fri, 05 Nov 2010 23:30:43 GMT
optimize join followed by a groupby
-----------------------------------

                 Key: HIVE-1772
                 URL: https://issues.apache.org/jira/browse/HIVE-1772
             Project: Hive
          Issue Type: Improvement
          Components: Query Processor
            Reporter: Namit Jain


explain SELECT x.key, count(1) FROM src1 x JOIN src y ON (x.key = y.key) group by x.key;


STAGE DEPENDENCIES:
  Stage-1 is a root stage
  Stage-2 depends on stages: Stage-1
  Stage-0 is a root stage


The above query issues 2 map-reduce jobs. 
The first MR job performs the join, whereas the second MR performs the group by.
Since the data is already sorted, the group by can be performed in the reducer of the join
itself.


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