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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