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https://issues.apache.org/jira/browse/HIVE-931?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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He Yongqiang updated HIVE-931:
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Attachment: hive-931-2009-12-03.patch
Attached a new patch. Had a lot of offline discussions with Namit. Thanks Namit!
Finally, we changed to rule to,
we will transform a group by to a sort based group by when
1) If a table's sort columns are empty, and buckets columns contains and only
contains all group by columns (order does not matter).
or
2) If a table's sort columns are not empty, group by columns are a prefix
subset of sort columns.
For example, if sorted by a,b,c, group by
a,
a,b
b,a
a,b,c
b,a,c ..
are all ok.
> 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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