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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 Tue, 15 Jun 2010 18:13:23 GMT

    [ https://issues.apache.org/jira/browse/HIVE-1139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12879051#action_12879051
] 

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

right... Ning is there any open source serializers/deserializers that you are aware of that
is reflections based, if so I can quickly implement a similar persistent map around that....

> 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
>          Components: Query Processor
>    Affects Versions: 0.5.0
>            Reporter: Ning Zhang
>            Assignee: Arvind Prabhakar
>         Attachments: PersistentMap.zip
>
>
> 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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