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https://issues.apache.org/jira/browse/HIVE-1139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12876979#action_12876979
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Arvind Prabhakar commented on HIVE-1139:
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I did some preliminary analysis for this JIRA and converted the
{{HashMapWrapper}} to implement the {{java.util.Map}} interface. This required
some changes all the way down to the underlying JDBM classes.
However, this alone is not sufficient to plug it into the {{GroupByOperator}}
implementation because the data stored in the {{HashMap}} is a mix of
serializable Java objects as well as {{Writable}}s. Since {{Writable}}s cannot
be directly serialized to Java, it follows that inorder to use this for fixing
the memory problem we need _an external serialization_ mechanism that can
handle arbitrary mixed type object graphs.
A trivial approach to address this would be to implement custom serialization
using Java reflection but that would incur cost of excessive reflection and
byte handling/marshaling.
If you have any other ideas regarding this, please add it to the comments of
this issue for consideration.
> 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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