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https://issues.apache.org/jira/browse/HIVE-6518?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13915238#comment-13915238
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Gopal V commented on HIVE-6518:
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Yes, also the ORC scenario is more complex for strings in dictionaries.
A substring does not drop the rest of the data off the memory overhead because
in vectorized mode, only the start:len get modified, no new allocations are
made.
So a group by SUBSTR() will keep the entire string in memory, except the VGBY
does not know that it does.
> Add a GC canary to the VectorGroupByOperator to flush whenever a GC is
> triggered
> --------------------------------------------------------------------------------
>
> Key: HIVE-6518
> URL: https://issues.apache.org/jira/browse/HIVE-6518
> Project: Hive
> Issue Type: Bug
> Components: Query Processor
> Affects Versions: 0.13.0
> Reporter: Gopal V
> Assignee: Gopal V
> Priority: Minor
> Attachments: HIVE-6518.1-tez.patch
>
>
> The current VectorGroupByOperator implementation flushes the in-memory hashes
> when the maximum entries or fraction of memory is hit.
> This works for most cases, but there are some corner cases where we hit GC
> ovehead limits or heap size limits before either of those conditions are
> reached due to the rest of the pipeline.
> This patch adds a SoftReference as a GC canary. If the soft reference is
> dead, then a full GC pass happened sometime in the near past & the
> aggregation hashtables should be flushed immediately before another full GC
> is triggered.
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