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From "Ning Zhang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-1139) GroupByOperator sometimes throws OutOfMemory error when there are too many distinct keys
Date Thu, 10 Jun 2010 03:47:13 GMT

    [ https://issues.apache.org/jira/browse/HIVE-1139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12877303#action_12877303
] 

Ning Zhang commented on HIVE-1139:
----------------------------------

Arvind, I remember I got this problem (non-serializable) problem before and the problem boils
down to support ObjectInspector in the same way as RowContainer does. In terms of a complete
interface of Map, it would be good to have but I would be very cautious about performance
penalty it may add. If the new API is not needed for now, I would vote for the ObjectInspector
support first. 

> 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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