fix: Do not mark GROUP BY derived unique keys as nullable#23636
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GROUP BY treats NULLs as equal, so every key combination -- NULL included -- occurs in exactly one output row, like a primary key. Marking the dependency nullable (multiple NULL keys may coexist) was overly conservative. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
`ReplaceDistinctWithAggregate` removes `Distinct::All` when a
`Dependency::Single` functional dependence covers all input fields.
However a nullable UNIQUE constraint also produces a `Single`-mode
dependence, and SQL UNIQUE permits multiple NULL keys. DISTINCT treats
NULLs as equal, so it must still collapse those rows:
CREATE TABLE t (x INT UNIQUE) AS VALUES (NULL), (NULL), (1);
SELECT DISTINCT x FROM t;
returned two NULL rows.
Only remove the DISTINCT when the dependence is non-nullable or none of
its source fields are actually nullable, mirroring the guards already
used by `EliminateJoin` and `Filter::is_scalar`.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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July 16, 2026 14:14
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Which issue does this PR close?
Rationale for this change
This query is wrong
What changes are included in this PR?
Fix bug by correctly computing and consuming
FunctionalDependenciesAre these changes tested?
Yes, new sqllogictest cases in
group_by.sltcover the wrong-results query, the plan for the nullableUNIQUEcase (theDISTINCTis planned as anAggregate), and thatDISTINCTover aPRIMARY KEYis still removed.Are there any user-facing changes?
No API changes.
SELECT DISTINCTover a table with a nullableUNIQUEconstraint now returns correct results (at the cost of keeping the aggregation in the plan).