Liya Fan created CALCITE-4132:
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Summary: 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
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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