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Date: Tue, 3 Mar 2009 23:17:56 -0800 (PST)
From: "Namit Jain (JIRA)" <jira@apache.org>
To: hive-dev@hadoop.apache.org
Subject: [jira] Commented: (HIVE-223) when using map-side aggregates -
 perform single map-reduce group-by
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    [ https://issues.apache.org/jira/browse/HIVE-223?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12678620#action_12678620 ] 

Namit Jain commented on HIVE-223:
---------------------------------

My mistake - merge is directly called (without iterate) in case of single map-reduce job with no map-side aggregations.
(hive.map.aggr=false and skew=false)

> when using map-side aggregates - perform single map-reduce group-by
> -------------------------------------------------------------------
>
>                 Key: HIVE-223
>                 URL: https://issues.apache.org/jira/browse/HIVE-223
>             Project: Hadoop Hive
>          Issue Type: Improvement
>          Components: Query Processor
>            Reporter: Joydeep Sen Sarma
>            Assignee: Namit Jain
>         Attachments: 223.2.txt, 223.3.txt, 223.patch1.txt, patch.txt
>
>
> today even when we do map side aggregates - we do multiple map-reduce jobs. however - the reason for doing multiple map-reduce group-bys (for single group-bys) was the fear of skews. When we are doing map side aggregates - skews should not exist for the most part. There can be two reason for skews:
> - large number of entries for a single grouping set - map side aggregates should take care of this
> - badness in hash function that sends too much stuff to one reducer - we should be able to take care of this by having good hash functions (and prime number reducer counts)
> So i think we should be able to do a single stage map-reduce when doing map-side aggregates.

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