[jira] [Commented] (CALCITE-3836) The hash codes of RelNodes are unreliable

["Julian Hyde (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CALCITE-3836?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17047291#comment-17047291 ] 

Julian Hyde commented on CALCITE-3836:
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Where does it say that the hash code may change? I see the following:

bq. Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.

> 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. 



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