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https://issues.apache.org/jira/browse/HIVE-609?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12732817#action_12732817
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Namit Jain commented on HIVE-609:
---------------------------------
incorporated comments
> optimize multi-group by
> ------------------------
>
> Key: HIVE-609
> URL: https://issues.apache.org/jira/browse/HIVE-609
> Project: Hadoop Hive
> Issue Type: Improvement
> Components: Query Processor
> Reporter: Namit Jain
> Assignee: Namit Jain
> Attachments: hive.609.1.patch, hive.609.2.patch, hive.609.3.patch
>
>
> For query like:
> from src
> insert overwrite table dest1 select col1, count(distinct colx) group by col1
> insert overwrite table dest2 select col2, count(distinct colx) group by col2;
> If map side aggregation is turned off, we currently do 4 map-reduce jobs.
> The plan can be optimized by running it in 3 map-reduce jobs, by spraying
> over the
> distinct column first and then aggregating individual results.
> This may not be possible if there are multiple distinct columns, but the
> above query is very common
> in data warehousing environments.
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