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https://issues.apache.org/jira/browse/HIVE-11638?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14711300#comment-14711300
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Jesus Camacho Rodriguez commented on HIVE-11638:
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+1 (pending tests)
> ExprNodeDesc hashMap accidentally degrades into O(N) instead of O(1)
> --------------------------------------------------------------------
>
> Key: HIVE-11638
> URL: https://issues.apache.org/jira/browse/HIVE-11638
> Project: Hive
> Issue Type: Improvement
> Components: Logical Optimizer
> Affects Versions: 0.14.0, 1.0.1, 1.1.1, 1.3.0, 1.2.1, 2.0.0
> Reporter: Gopal V
> Assignee: Gopal V
> Labels: Performance
> Attachments: HIVE-11638.1.patch
>
>
> Due to the absence of ExprNodeDesc::equals() & the presence of a hashCode(),
> any code which computes a Set or HashMap of identical objects ends up with a
> skewed hashmap which degrades column lookups from O(1) to O(N) during
> optimizer passes.
> So, several hundred references to a single column desc will be stored in a
> single hashbucket, but since they default to Object::equals(), they are not
> equal to each other.
> Finalize the equals() method, so that the approach from HIVE-10215 carries
> over safely.
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