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release-21.1: opt: do not cross-join input of semi-join #78976

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merged 2 commits into from
Mar 29, 2022
Merged

release-21.1: opt: do not cross-join input of semi-join #78976

merged 2 commits into from
Mar 29, 2022

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mgartner
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@mgartner mgartner commented Mar 29, 2022 •
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Backport:

Please see individual PRs for details.

/cc @cockroachdb/release

Release justification: This is a minimal fix for a bug that causing incorrect
query results.

@mgartner
opt: do not cross-join input of semi-join
0951209 
This commit fixes a logical correctness bug caused when
`GenerateLookupJoins` cross-joins the input of a semi-join with a set of
constant values to constrain the prefix columns of the lookup index. The
cross-join is an invalid transformation because it increases the size of
the join's input and can increase the size of the join's output.

We already avoid these cross-joins for left and anti-joins (see cockroachdb#59646).
When addressing those cases, the semi-join case was incorrectly assumed
to be safe.

Fixes cockroachdb#78681

Release note (bug fix): A bug has been fixed which caused the optimizer
to generate invalid query plans which could result in incorrect query
results. The bug, which has been present since version 21.1.0, can
appear if all of the following conditions are true: 1) the query
contains a semi-join, such as queries in the form:
`SELECT * FROM t1 WHERE EXISTS (SELECT * FROM t2 WHERE t1.a = t2.a);`,
2) the inner table has an index containing the equality column, like
`t2.a` in the example query, 3) the index contains one or more
columns that prefix the equality column, and 4) the prefix columns are
`NOT NULL` and are constrained to a set of constant values via a `CHECK`
constraint or an `IN` condition in the filter.
@mgartner mgartner requested review from rytaft and a team March 29, 2022 17:59
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blathers-crl bot commented Mar 29, 2022

Thanks for opening a backport.

Please check the backport criteria before merging:

  • Patches should only be created for serious issues or test-only changes.
  • Patches should not break backwards-compatibility.
  • Patches should change as little code as possible.
  • Patches should not change on-disk formats or node communication protocols.
  • Patches should not add new functionality.
  • Patches must not add, edit, or otherwise modify cluster versions; or add version gates.
If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.
  • There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
  • The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
  • New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
  • The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

  • What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
  • Will this work in a cluster of mixed patch versions? Did we test that?
  • If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

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rytaft
rytaft approved these changes Mar 29, 2022
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:lgtm:

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: :shipit: complete! 1 of 0 LGTMs obtained (waiting on @mgartner)

@mgartner
sql: deflake unique logic test
772cc86 
PR cockroachdb#78685 changes the query plan of one query in a logic test in a way
that makes the test flakey. This commit guarantees that the test cannot
be flakey, regardless of the query plan.

Release note: None
@mgartner mgartner merged commit 7bf15f3 into cockroachdb:release-21.1 Mar 29, 2022
@mgartner mgartner deleted the backport21.1-78685 branch March 29, 2022 22:44
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@mgartner @cockroach-teamcity @rytaft