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https://issues.apache.org/jira/browse/CALCITE-3836?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17049972#comment-17049972
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Xiening Dai commented on CALCITE-3836:
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[~danny0405] this article says - "simply asking for the identity hash code of
an object will disable biased locking". This is because you cannot store both
hash code and thread id in the mark word. So calling object.hashCode() would
transfer it into unbiased mode. You get the same behavior even if you choose to
return RelNode id. It has nothing to do with the hash algorithm, but just an
artifact of the implementation of biased lock.
> The hash codes of RelNodes are unreliable
> -----------------------------------------
>
> Key: CALCITE-3836
> URL: https://issues.apache.org/jira/browse/CALCITE-3836
> Project: Calcite
> Issue Type: Bug
> Components: core
> Reporter: Liya Fan
> Priority: Critical
> Labels: pull-request-available
> Time Spent: 1h
> Remaining Estimate: 0h
>
> For all sub-classes of AbstractRelNode, the {{hashCode}} methods depend on
> {{AbstractRelNode#hashCode}}, because it is declared as final.
> {{AbstractRelNode#hashCode}} depends on {{Object#hashCode}}, which is called
> identify hash code. The details of identity hash code depends on the specific
> JVM implementation. For many JVMs, the implementation is based on the object
> address in the memory. The problem is that, the address of an object may
> change in a JVM, due to GC, memory contraction, etc. So the hash code of an
> object may change, even if the content of the object is not changed (This can
> be confirmed from the JavaDoc of {{Object#hashCode}}).
> This problem may cause severe issues that are hard to diagnose and debug,
> like an object is in the hash table, but cannot be retrieved; duplicate
> objects in the hash map, etc.
> To solve the problem, we compute a hash code solely from the node id. This is
> consistent with the previous semantics, and solves the above problem.
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