[jira] [Commented] (CALCITE-4096) Change Pair.hashCode() not to use XOR

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

    [ https://issues.apache.org/jira/browse/CALCITE-4096?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17148814#comment-17148814 ] 

Julian Hyde commented on CALCITE-4096:
--------------------------------------

It's a trade-off. On balance I prefer what we have.

> Change Pair.hashCode() not to use XOR
> -------------------------------------
>
>                 Key: CALCITE-4096
>                 URL: https://issues.apache.org/jira/browse/CALCITE-4096
>             Project: Calcite
>          Issue Type: Improvement
>          Components: core
>            Reporter: Haisheng Yuan
>            Priority: Major
>
> JDK Map Entry uses XOR maybe because it is unlikely to use Map Entry as the HashMap key.
> But Pair in Calcite is a general data structure, it is used in several places as the key of HashMap/HashSet. XOR is not a good candidate for hash algorithm, it is more likely to generate hash collision than simple prime multiplication, especially when pair.left equals pair.right.



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