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From "Zheng Shao (JIRA)" <>
Subject [jira] Commented: (HIVE-609) optimize multi-group by
Date Mon, 20 Jul 2009 16:51:14 GMT


Zheng Shao commented on HIVE-609:

This patch is generated in the test results directory, so it does not contain code changes.

> optimize multi-group by 
> ------------------------
>                 Key: HIVE-609
>                 URL:
>             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, hive.609.4.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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