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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: 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, 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 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.