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Posted to issues-all@impala.apache.org by "Riza Suminto (Jira)" <ji...@apache.org> on 2023/06/09 20:06:00 UTC

[jira] [Created] (IMPALA-12200) Cap stats NDV from SetOperationStmt.createMetadata

Riza Suminto created IMPALA-12200:
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             Summary: 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


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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