[
https://issues.apache.org/jira/browse/HIVE-931?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12780459#action_12780459
]
Namit Jain commented on HIVE-931:
---------------------------------
I think: bucketCols should be same as groupbyCols. not a superset.
Consider:
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 sortedCols
> 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
>
>
> 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.
