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From "Namit Jain (JIRA)" <>
Subject [jira] Commented: (HIVE-931) Sorted Group By
Date Fri, 20 Nov 2009 06:09:39 GMT


Namit Jain commented on HIVE-931:

I think: bucketCols should be same as groupbyCols. not a superset.


select ... group by a,b:

where data is bucketed by a,b,c.

A mapper might get:

a1   b1   c1
a1   b2   c2
a1   b1   c3

in which case the current algorithm might not work.

Also, change the name of the variable in isTableSortedbyColumns to bucketedCols instead of

> Sorted Group By
> ---------------
>                 Key: HIVE-931
>                 URL:
>             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
> If the table is sorted by a given key, we don't use that for group by. That can be very
> 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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