[jira] Updated: (HIVE-223) when using map-side aggregates - perform single map-reduce group-by

["Namit Jain (JIRA)" <ji...@apache.org>]

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

Namit Jain updated HIVE-223:
----------------------------

    Attachment: 223.patch1.txt

> 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.patch1.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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