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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:
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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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