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https://issues.apache.org/jira/browse/CALCITE-4132?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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ASF GitHub Bot updated CALCITE-4132:
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Labels: pull-request-available (was: )
> Estimate the NDV accurately
> ---------------------------
>
> Key: CALCITE-4132
> URL: https://issues.apache.org/jira/browse/CALCITE-4132
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: Liya Fan
> Assignee: Liya Fan
> Priority: Major
> Labels: pull-request-available
> Time Spent: 10m
> Remaining Estimate: 0h
>
> Currently, we estimate the NDV of many operators based on the
> RelMdUtil#numDistinctVals method. This method estimates the expected number
> of distinct values selected n times (with replacement) from a collection with
> N distinct values. The estimation is based on the approximation when N
> approaches infinity.
> However, when N is not a large number, the difference between the approximate
> and exact values can be notabe. In addtion, the error can be magnified by
> different combinations of N and n, which can lead the optimizer to make wrong
> decisions.
> Therefore, we give the exact estimation based on the unbiased estimator (The
> proof is given in the comment).
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