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From "Zheng Shao (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HIVE-223) when using map-side aggregates - perform single map-reduce group-by
Date Sat, 07 Mar 2009 01:36:56 GMT

     [ https://issues.apache.org/jira/browse/HIVE-223?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Zheng Shao updated HIVE-223:
----------------------------

       Resolution: Fixed
    Fix Version/s: 0.3.0
     Release Note: 
HIVE-223. Single map-reduce group-by using map-side aggregation. (Namit Jain via zshao)

     Hadoop Flags: [Reviewed]
           Status: Resolved  (was: Patch Available)

Committed revision 751166. Thanks Namit!


> 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
>             Fix For: 0.3.0
>
>         Attachments: 223.2.txt, 223.3.txt, 223.patch1.txt, hive.223.4.patch, 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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