hive-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Zheng Shao (JIRA)" <>
Subject [jira] Commented: (HIVE-931) Sorted Group By
Date Fri, 04 Dec 2009 07:01:21 GMT


Zheng Shao commented on HIVE-931:

Is the "sorted by" property always the same as "bucketed by" for a table?

The name "hive.optimize.groupby" is a bit too general but I guess it's OK for now. Can we
explain what is "bucketed group by" in the hive-default.xml? Users probably won't understand
what it is.

> 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, hive-931-2009-12-03.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.

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

View raw message