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https://issues.apache.org/jira/browse/FLINK-11964?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17007246#comment-17007246
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Jingsong Lee commented on FLINK-11964:
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PR: [https://github.com/apache/flink/pull/10756]
> Avoid hash collision of partition and bucket in HybridHashTable in Blink SQL
> ----------------------------------------------------------------------------
>
> Key: FLINK-11964
> URL: https://issues.apache.org/jira/browse/FLINK-11964
> Project: Flink
> Issue Type: Bug
> Components: Runtime / Task
> Reporter: Jingsong Lee
> Priority: Major
> Fix For: 1.10.0
>
>
> In HybridHashTable, first select the corresponding partition according to
> hashCode, and then select the bucket in the partition according to hashCode,
> using the same hashCode can easily cause hash collision.
> Consider doing some mix to hashCode when choosing bucket.
> Like JDK HashMap, we can just XOR some shifted bits in the cheapest possible
> way to reduce systematic lossage, as well as to incorporate impact of the
> highest bits that would otherwise never be used in index calculations because
> of table bounds. (bucket use power-of-two masking). Just like: (hash ^
> (hash >>> 16))
> In some cases, if a lot of conflicts occurred, this will lead to job hang,
> because hash join will degenerate to nested loop join.
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