[
https://issues.apache.org/jira/browse/CALCITE-3836?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17047324#comment-17047324
]
Liya Fan commented on CALCITE-3836:
-----------------------------------
[~julianhyde] Sorry it did not directly say that hash code may change. It only
says "This is typically implemented by converting the internal address of the
object into an integer, but this implementation technique is not required by
the Java programming language."
An article discussing this problem can be found in
https://srvaroa.github.io/jvm/java/openjdk/biased-locking/2017/01/30/hashCode.html
> 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: 10m
> 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.
--
This message was sent by Atlassian Jira
(v8.3.4#803005)
