Wrong results: `ORDER BY ... NULLS LAST` on a running `MAX` window function puts the NULL row first

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#27,267 4 comentarios 0 reacciones 1 asignado Ver en GitHub

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@itang06 ya está trabajando en esto.

Desde el 11/11/2026.

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Descripción

bug
Describe the bug

A query that computes MAX(x) OVER (ORDER BY ts) and then sorts on that column with ASC NULLS LAST returns rows in the wrong order when x contains NULLs. The NULL row is returned first, where the query asked for it last.

The same wrong order appears for:

  • MIN(x) OVER (ORDER BY ts) sorted DESC NULLS LAST
  • MAX(x) OVER (ORDER BY ts ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING) sorted DESC NULLS FIRST
To Reproduce
set datafusion.execution.target_partitions = 1;

CREATE TABLE t(ts INT, x INT) AS VALUES (1, NULL), (2, 3), (3, 1), (4, 5);

SELECT ts, m
FROM (
  SELECT ts, MAX(x) OVER (ORDER BY ts) AS m
  FROM (SELECT * FROM t ORDER BY ts)
)
ORDER BY m ASC NULLS LAST;

Returns:

+----+------+
| ts | m    |
+----+------+
| 1  | NULL |     <-- NULL is first, not last!
| 2  | 3    |
| 3  | 3    |
| 4  | 5    |
+----+------+
Expected behavior

NULLS LAST means the NULL row comes after every non-NULL value:

+----+------+
| ts | m    |
+----+------+
| 2  | 3    |
| 3  | 3    |
| 4  | 5    |
| 1  | NULL |
+----+------+
Additional context

EXPLAIN shows the final sort on m is removed, so the planner believes the window output already satisfies m ASC NULLS LAST. Reproduced on main at 94486b1e93.

Found by @jayzhan211 while reviewing the EquivalenceProperties docs in #27250. The original report and repro are in this review comment: https://github.com/apache/datafusion/pull/27250#discussion_r4512590554

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