opt: fix min cardinality calculation of EXCEPT #89108
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Prior to this commit, we assumed that the minimum cardinality of EXCEPT could never be lower than the minimum cardinality of the left side minus the maximum cardinality of the right side. However, this assumption is only true for EXCEPT ALL, not EXCEPT. For example, VALUES (1), (1) EXCEPT VALUES (1) produces 0 rows, even though the left side produces more rows than the right. Because of this invalid assumption, the optimizer could make some invalid transformations, resulting in incorrect results. Fixes cockroachdb#89101 Release note (bug fix): Fixed a bug that has existed since v2.1.0 where queries containing a subquery with EXCEPT could produce incorrect results. This could happen if the optimizer could guarantee that the left side of the EXCEPT always returned more rows than the right side. In this case, the optimizer made a faulty assupmtion that the EXCEPT subquery always returned at least one row, which could cause the optimizer to perform an invalid transformation, possibly causing the full query to return incorrect results.
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DrewKimball
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Sep 30, 2022
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complete! 1 of 0 LGTMs obtained (waiting on @mgartner)
mgartner
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Sep 30, 2022
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Reviewed 2 of 2 files at r1, all commit messages.
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TFTRs! bors r+ |
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Build succeeded: |
This was referenced Sep 30, 2022
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blathers backport 22.2.0 |
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Prior to this commit, we assumed that the minimum cardinality of
EXCEPTcould never be lower than the minimum cardinality of the left side minus the maximum cardinality of the right side. However, this assumption is only true forEXCEPT ALL, notEXCEPT.For example,
VALUES (1), (1) EXCEPT VALUES (1)produces 0 rows, even though the left side produces more rows than the right. Because of this invalid assumption, the optimizer could make some invalid transformations, resulting in incorrect results.Fixes #89101
Release note (bug fix): Fixed a bug that has existed since v2.1.0 where queries containing a subquery with
EXCEPTcould produce incorrect results. This could happen if the optimizer could guarantee that the left side of theEXCEPTalways returned more rows than the right side. In this case, the optimizer made a faulty assupmtion that theEXCEPTsubquery always returned at least one row, which could cause the optimizer to perform an invalid transformation, possibly causing the full query to return incorrect results.