[jira] Commented: (HIVE-1139) GroupByOperator sometimes throws OutOfMemory error when there are too many distinct keys

["Ning Zhang (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-1139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12831101#action_12831101 ] 

Ning Zhang commented on HIVE-1139:
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GroupByOperator uses HashMap's size() and iterator() API, which is not supported by HashMapWrapper right now. We should extended HashMapWrapper to support those. 

> 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: Ning Zhang
>
> 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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