[ https://issues.apache.org/jira/browse/CALCITE-4132?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Andrei Sereda closed CALCITE-4132. ---------------------------------- Released as part of 1.25 ([68b02dfd4af15bc|https://github.com/apache/calcite/commit/68b02dfd4af15bc94a91a0cd2a30655d04439555]) > Estimate the number of distinct values more 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 > Fix For: 1.25.0 > > Time Spent: 1h 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. > For example, when we select one element from a table with a 4-value enum, we > expect to get one distinct value according to common sense. However, the > current implementation gives 0.88, which is counter-intuitive, and leads to a > 10+% error. > Therefore, we give the exact estimation based on the unbiased estimator (The > proof is given in the comment). -- This message was sent by Atlassian Jira (v8.3.4#803005)