Incorrect results: ORDER BY drops a sort key based on a nullable UNIQUE constraint #23818

Is your feature request related to a problem or challenge?

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

A trailing sort key is dropped when an earlier key functionally determines 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.

CREATE TABLE t (x INT UNIQUE, y INT) AS VALUES (NULL, 2), (NULL, 1), (1, 3);
SELECT x, y FROM t ORDER BY x NULLS LAST, y;
-- returns:
-- 1    3
-- NULL 2
-- NULL 1
--
-- should be:
-- 1    3
-- NULL 1
-- NULL 2

The plan shows the y sort key is gone entirely:

EXPLAIN SELECT x, y FROM t ORDER BY x NULLS LAST, y;
-- logical_plan
-- 01)Sort: t.x ASC NULLS LAST            <-- `y` dropped
-- 02)--TableScan: t projection=[x, y]
-- physical_plan
-- 01)SortExec: expr=[x@0 ASC NULLS LAST], preserve_partitioning=[false]
-- 02)--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, y from t order by x nulls last, y;"
┌───────┬───────┐
│   x   │   y   │
│ int32 │ int32 │
├───────┼───────┤
│     1 │     3 │
│  NULL │     1 │
│  NULL │     2 │
└───────┴───────┘

Note the two NULL rows are stored with y descending on purpose — with them in ascending order the answer comes out right by accident, even though the plan is equally wrong.

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Fix the bug.

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Additional context

Related:

• Incorrect results with unique constraints #23634
• 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