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From "Soundararajan Velu (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 17:34:13 GMT

    [ https://issues.apache.org/jira/browse/HIVE-1139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12877500#action_12877500
] 

Soundararajan Velu commented on HIVE-1139:
------------------------------------------

Ning, Aravind, I got this implemented and it looks good so far, I will try uploading the version
I have modified after a thorough test, All I did was copy the HashMap implementation into
the HashMapWrapper (leaving the existing functionality intact), now HashmapWrapper works exactly
like hashmap, but I did not get to test out the serialization issues. will do that and update
you guys. I think this should help us in our OOM issue around GroupBy...

> 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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