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https://issues.apache.org/jira/browse/IMPALA-12200?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Work on IMPALA-12200 started by Riza Suminto.
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> Cap stats NDV from SetOperationStmt.createMetadata
> --------------------------------------------------
>
> Key: IMPALA-12200
> URL: https://issues.apache.org/jira/browse/IMPALA-12200
> Project: IMPALA
> Issue Type: Improvement
> Components: Frontend
> Affects Versions: Impala 4.3.0
> Reporter: Riza Suminto
> Assignee: Riza Suminto
> Priority: Major
>
> Union operator will create merged ColumnStats at
> SetOperationStmt.createMetadata where it adds all ColumnStats from its input
> children. One of the stats being accumulated is NDV (num distinct value).
> There is an opportunity to lower the resulting NDV if all source expression
> is referring to the same column. This lower NDV can benefit Aggregation node
> on top of the Union node because it can lower cardinality and memory estimate
> of the Aggregation node.
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