Incorrect results: GROUP BY column dropped based on a nullable UNIQUE constraint #23819

Is your feature request related to a problem or challenge?

Follow-up to #23634 and #23818: the same nullable-UNIQUE confusion also affects GROUP BY.

A grouping column is dropped when the remaining grouping columns already determine it. That is valid for a PRIMARY KEY, but not for a UNIQUE column, which permits multiple NULL rows and therefore does not determine anything across them. Dropping it merges groups that should stay separate, so rows go missing from the result.

CREATE TABLE t (x INT UNIQUE, y INT) AS VALUES (NULL, 2), (NULL, 1), (1, 3);
SELECT x FROM t GROUP BY x, y;
-- returns 2 rows:
-- 1
-- NULL
--
-- should return 3 rows (the two NULL rows differ in `y`):
-- 1
-- NULL
-- NULL

The plan shows y is gone from the GROUP BY entirely:

EXPLAIN SELECT x FROM t GROUP BY x, y;
-- logical_plan
-- 01)Aggregate: groupBy=[[t.x]], aggr=[[]]     <-- `y` dropped
-- 02)--TableScan: t projection=[x]
-- physical_plan
-- 01)AggregateExec: mode=FinalPartitioned, gby=[x@0 as x], aggr=[]
-- 02)--RepartitionExec: partitioning=Hash([x@0], 16), input_partitions=1
-- 03)----AggregateExec: mode=Partial, gby=[x@0 as x], aggr=[]
-- 04)------DataSourceExec: partitions=1, partition_sizes=[1]

Verified against duckdb:

❯ duckdb -c "create table t (x int unique, y int); insert into t values (null,2),(null,1),(1,3); select x from t group by x, y;"
┌───────┐
│   x   │
│ int32 │
├───────┤
│  NULL │
│  NULL │
│     1 │
└───────┘

Note that y has to be absent from the select list to trigger this — SELECT x, y FROM t GROUP BY x, y returns the correct three rows.

Describe the solution you'd like

Fix the bug.

Describe alternatives you've considered

Additional context

Related:

• Incorrect results with unique constraints #23634
• Incorrect results: ORDER BY drops a sort key based on a nullable UNIQUE constraint #23818
• fix: do not remove DISTINCT when a unique key was downgraded by a join #23548
• fix: Do not mark GROUP BY derived unique keys as nullable #23636