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From "Arvind Prabhakar (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-1139) GroupByOperator sometimes throws OutOfMemory error when there are too many distinct keys
Date Wed, 09 Jun 2010 21:47:13 GMT

    [ https://issues.apache.org/jira/browse/HIVE-1139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12877222#action_12877222
] 

Arvind Prabhakar commented on HIVE-1139:
----------------------------------------

If there is interest, I can file a separate JIRA for modifying {{HashMapWrapper}} to support
the {{java.util.Map}} interface and decouple that work from this JIRA. I think there is a
lot of benefit in doing just that. Also, we could have this JIRA depend upon that as a prerequisite.



> GroupByOperator sometimes throws OutOfMemory error when there are too many distinct keys
> ----------------------------------------------------------------------------------------
>
>                 Key: HIVE-1139
>                 URL: https://issues.apache.org/jira/browse/HIVE-1139
>             Project: Hadoop Hive
>          Issue Type: Bug
>            Reporter: Ning Zhang
>            Assignee: Arvind Prabhakar
>
> When a partial aggregation performed on a mapper, a HashMap is created to keep all distinct
keys in main memory. This could leads to OOM exception when there are too many distinct keys
for a particular mapper. A workaround is to set the map split size smaller so that each mapper
takes less number of rows. A better solution is to use the persistent HashMapWrapper (currently
used in CommonJoinOperator) to spill overflow rows to disk. 

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