[jira] Commented: (HIVE-931) Sorted Group By

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

    [ https://issues.apache.org/jira/browse/HIVE-931?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12780221#action_12780221 ] 

Namit Jain commented on HIVE-931:
---------------------------------

Discussed with Yongqiang offline - this should be a optimization step instead of the current approach.
Since, at that time, partition pruning has also been performed

> 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
>
>
> 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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