[
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.
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
