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https://issues.apache.org/jira/browse/HIVE-22455?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16967155#comment-16967155
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Julian Hyde commented on HIVE-22455:
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There are a bunch of rules in Calcite's class PruneEmptyRules that recognize
empty relational expressions and simplify accordingly.
> Union branch removal rule does not kick in.
> -------------------------------------------
>
> Key: HIVE-22455
> URL: https://issues.apache.org/jira/browse/HIVE-22455
> Project: Hive
> Issue Type: Improvement
> Reporter: Steve Carlin
> Priority: Major
>
> After the Calcite upgrade to 1.21, there is a rule where 2 branches of a
> union have limit 0. This can be simplified.
> This can be found in: union_assertion_type.q.out
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