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https://issues.apache.org/jira/browse/HIVE-1772?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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John Sichi reassigned HIVE-1772:
--------------------------------
Assignee: Navis
> 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
> Assignee: Navis
> Attachments: HIVE-1772.1.patch
>
>
> 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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