Zoltan Haindrich created HIVE-22238:
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Summary: PK/FK selectivity estimation underscales estimations
Key: HIVE-22238
URL: https://issues.apache.org/jira/browse/HIVE-22238
Project: Hive
Issue Type: Bug
Reporter: Zoltan Haindrich
Assignee: Zoltan Haindrich
at [this
point|https://github.com/apache/hive/blob/5098d155a1e6a164253f5fa98755273bc34085df/ql/src/java/org/apache/hadoop/hive/ql/optimizer/stats/annotation/StatsRulesProcFactory.java#L2182]
the parent operators rownum is scaled according to pkfkselectivity
however [pkfkselectivity is
computed|https://github.com/apache/hive/blob/5098d155a1e6a164253f5fa98755273bc34085df/ql/src/java/org/apache/hadoop/hive/ql/optimizer/stats/annotation/StatsRulesProcFactory.java#L2157]
on a whole subtree.
Scaling it by that amount will count in estimation already used when
parentstats was calculated...so depending on the number of upstream joins -
this may lead to severe underestimations
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