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