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From "He Yongqiang (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HIVE-931) Sorted Group By
Date Wed, 02 Dec 2009 06:04:20 GMT

     [ https://issues.apache.org/jira/browse/HIVE-931?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

He Yongqiang updated HIVE-931:
------------------------------

    Attachment: hive-931-2009-12-01.patch

updated the patch. And put sort columns into consideration.

     * We use bucket columns only when the sorted column set is empty or the
     * sorted column set is an exact prefix match of bucket columns. For example, A
     * table is bucketed by column a,b, and c, and a query wants to group by
     * a,b,c. If the table's sort column is null, or is [a],[a,b], or [a,b,c],
     * we can use the 'sorted groupby' by looking at the bucket columns .
     * 
     * If we can can not determine by looking at bucketed columns and the table
     * has sort columns, we resort to sort columns. We can use bucket group by
     * if the groupby column set is an exact prefix match of sort columns.

> 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
>
>
> 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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