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https://issues.apache.org/jira/browse/HIVE-535?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12836832#action_12836832
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Ning Zhang commented on HIVE-535:
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Interesting indeed. I checked the license of these two open source projects and
they both turn out to be LGPL. So we probably can only use them as references
and implement our own Java/C++ version.
> Memory-efficient hash-based Aggregation
> ---------------------------------------
>
> Key: HIVE-535
> URL: https://issues.apache.org/jira/browse/HIVE-535
> Project: Hadoop Hive
> Issue Type: Improvement
> Affects Versions: 0.4.0
> Reporter: Zheng Shao
>
> Currently there are a lot of memory overhead in the hash-based aggregation in
> GroupByOperator.
> The net result is that GroupByOperator won't be able to store many entries in
> its HashTable, and flushes frequently, and won't be able to achieve very good
> partial aggregation result.
> Here are some initial thoughts (some of them are from Joydeep long time ago):
> A1. Serialize the key of the HashTable. This will eliminate the 16-byte
> per-object overhead of Java in keys (depending on how many objects there are
> in the key, the saving can be substantial).
> A2. Use more memory-efficient hash tables - java.util.HashMap has about 64
> bytes of overhead per entry.
> A3. Use primitive array to store aggregation results. Basically, the UDAF
> should manage the array of aggregation results, so UDAFCount should manage a
> long[], UDAFAvg should manage a double[] and a long[]. The external code
> should pass an index to iterate/merge/terminal an aggregation result. This
> will eliminate the 16-byte per-object overhead of Java.
> More ideas are welcome.
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