Fix ORCA GbAgg drop for nullable UNIQUE keys - #1941
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CXformSimplifyGbAgg::FDropGbAgg() drops a GROUP BY/DISTINCT's
aggregate node when the grouping columns cover a key derived from
a UNIQUE or PRIMARY KEY constraint, assuming the key already
guarantees one row per value.
That's wrong for a plain UNIQUE column: SQL allows multiple NULLs
in it (NULL is never "equal" for uniqueness checks), but dedup
must still collapse those NULLs into one row.
Before:
create table repro (c0 numeric unique);
insert into repro values (1), (2), (null), (null), (null);
explain (costs off) select distinct c0 from repro;
Gather Motion 3:1 (slice1; segments: 3)
-> Seq Scan on repro -- no aggregate, all 5 rows returned
After:
Gather Motion 3:1 (slice1; segments: 3)
-> GroupAggregate
Group Key: repro.c0
-> Seq Scan on repro -- correctly returns 1, 2, NULL
Fix by also requiring every key column to be provably NOT NULL
(CExpression::DeriveNotNullColumns()) before dropping the
aggregate. PRIMARY KEY columns are always NOT NULL, so that case
is unaffected.
yjhjstz
reviewed
Sep 1, 2026
| insert into t7 values (1, 1), (2, 2), (null, 3), (null, 4), (null, 5); | ||
| insert into t8 values (1, 1), (2, 2), (3, 3); | ||
| insert into t9 values (1, 1), (2, 2), (3, 3); | ||
| explain (costs off) select distinct a from t7; |
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CXformSimplifyGbAgg::FDropGbAgg() drops a GROUP BY/DISTINCT's aggregate node when the grouping columns cover a key derived from a UNIQUE or PRIMARY KEY constraint, assuming the key already guarantees one row per value.
That's wrong for a plain UNIQUE column: SQL allows multiple NULLs in it (NULL is never "equal" for uniqueness checks), but dedup must still collapse those NULLs into one row.
Before:
create table repro (c0 numeric unique);
insert into repro values (1), (2), (null), (null), (null);
explain (costs off) select distinct c0 from repro;
Gather Motion 3:1 (slice1; segments: 3)
-> Seq Scan on repro -- no aggregate, all 5 rows returned
After:
Gather Motion 3:1 (slice1; segments: 3)
-> GroupAggregate
Group Key: repro.c0
-> Seq Scan on repro -- correctly returns 1, 2, NULL
Fix by also requiring every key column to be provably NOT NULL (CExpression::DeriveNotNullColumns()) before dropping the aggregate. PRIMARY KEY columns are always NOT NULL, so that case is unaffected.
Fixes #ISSUE_Number
What does this PR do?
Type of Change
Breaking Changes
Test Plan
make installcheckmake -C src/test installcheck-cbdb-parallelImpact
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User-facing changes:
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