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From "Namit Jain (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-931) Sorted Group By
Date Sat, 21 Nov 2009 02:15:39 GMT

    [ https://issues.apache.org/jira/browse/HIVE-931?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12780886#action_12780886
] 

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

1. Add new parameter in hive-default.xml
2. Utilities.java: change function names - extractColumnNamesFromSortCols
   variable name: bucketCol: line 813
3. Remove all the tabs
4. Given	the fact that we are not doing	this optimization across sub-queries right now,
   would it be simpler to maintain the group by operator to table mapping via a separate walker
   instead of getting while generating the group by operator ? -- I am fine with	the current
approach
   also, but just a question.
5. You are still doing partition pruning in GroupByOptimizer - why cant we reuse the mapping
from
   ParseContext. That was the whole reason for storing it in ParseContext.


Sorry about being so picky...

> 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
>
>
> 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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