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From "He Yongqiang (JIRA)" <>
Subject [jira] Commented: (HIVE-931) Sorted Group By
Date Wed, 02 Dec 2009 07:11:20 GMT


He Yongqiang commented on HIVE-931:

Isnt there a bug here ?
group by foo(foo2(x))
You cant assume that the first child is a column - you should recurse till you get a column

Thanks, Namit. I think i did not assume that the first child is a column, i just inserted
all children exprs into the first place of groupByKeys list, and recurse.

Will add a testcase for this.

> 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, hive-931-2009-11-20.3.patch,
hive-931-2009-11-21.patch, hive-931-2009-12-01.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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