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From "He Yongqiang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-931) Sorted Group By
Date Fri, 04 Dec 2009 07:15:20 GMT

    [ https://issues.apache.org/jira/browse/HIVE-931?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12785796#action_12785796
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He Yongqiang commented on HIVE-931:
-----------------------------------

Hi Zheng,
bq. Is the "sorted by" property always the same as "bucketed by" for a table? 
They are not the same. And usually 'sorted by' is empty.

 bq. Can we explain what is "bucketed group by" in the hive-default.xml? Users probably won't
understand what it is.
Yes, i should add more explanation for this. I am ok to add it in a new jira, or update it
on wiki, or both. 


> Sorted Group By
> ---------------
>
>                 Key: HIVE-931
>                 URL: https://issues.apache.org/jira/browse/HIVE-931
>             Project: Hadoop Hive
>          Issue Type: New Feature
>          Components: Query Processor
>            Reporter: Namit Jain
>            Assignee: He Yongqiang
>             Fix For: 0.5.0
>
>         Attachments: hive-931-2009-11-18.patch, hive-931-2009-11-19.patch, hive-931-2009-11-20.3.patch,
hive-931-2009-11-21.patch, hive-931-2009-12-01.patch, hive-931-2009-12-03.patch
>
>
> If the table is sorted by a given key, we don't use that for group by. That can be very
useful.
> For eg: if T is sorted by column c1,
> For select c1, aggr() from T group by c1
> we always use a single map-reduce job. No hash table is needed on the mapper, since the
data is sorted by c1 anyway.
> This will reduce the memory pressure on the mapper and also remove overhead of maintaining
the hash table.

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