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Subject: BUG #19532: Window run-condition optimization can produce wrong
 results for count() window aggregates
To: pgsql-bugs@lists.postgresql.org
From: PG Bug reporting form <noreply@postgresql.org>
Cc: imchifan@163.com
Reply-To: imchifan@163.com, pgsql-bugs@lists.postgresql.org
Date: Wed, 24 Jun 2026 14:34:14 +0000
Message-ID: <19532-a8893c9d2a630a30@postgresql.org>
Auto-Submitted: auto-generated
Archived-At: 
 <https://www.postgresql.org/message-id/19532-a8893c9d2a630a30%40postgresql.org>
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The following bug has been logged on the website:

Bug reference:      19532
Logged by:          Qifan Liu
Email address:      imchifan@163.com
PostgreSQL version: 18.4
Operating system:   Ubuntu 20.04 x86-64, docker image postgres:18.4
Description:       =20

## PoC

```sql
DROP TABLE IF EXISTS t;

CREATE TABLE t (
    id int PRIMARY KEY,
    k  int NOT NULL,
    v  int
);

INSERT INTO t(id, k, v) VALUES
    (1, 1, 1),
    (2, 1, NULL),
    (3, 1, 1),
    (4, 2, 1);

WITH
source_rows AS (
    SELECT id
    FROM (
        SELECT id,
               count(v) OVER (
                   ORDER BY k
                   RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
                   EXCLUDE CURRENT ROW
               ) AS c
        FROM t
    ) s
    WHERE c <=3D 1
),
reference_rows AS (
    SELECT t1.id
    FROM t AS t1
    WHERE (
        SELECT count(t2.v)
        FROM t AS t2
        WHERE t2.k <=3D t1.k
          AND t2.id <> t1.id
    ) <=3D 1
),
metamorphic_rows AS (
    SELECT id
    FROM (
        SELECT id,
               count(v) OVER (
                   ORDER BY k
                   RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
                   EXCLUDE CURRENT ROW
               ) AS c
        FROM t
    ) s
    WHERE c + 0 <=3D 1
)
SELECT 'source' AS label, coalesce(array_agg(id ORDER BY id), '{}'::int[])
AS ids
FROM source_rows
UNION ALL
SELECT 'reference', coalesce(array_agg(id ORDER BY id), '{}'::int[])
FROM reference_rows
UNION ALL
SELECT 'metamorphic_no_pushdown', coalesce(array_agg(id ORDER BY id),
'{}'::int[])
FROM metamorphic_rows;

EXPLAIN (COSTS OFF)
SELECT id
FROM (
    SELECT id,
           count(v) OVER (
               ORDER BY k
               RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
               EXCLUDE CURRENT ROW
           ) AS c
    FROM t
) s
WHERE c <=3D 1;
```

## Expected Behavior

The source query should return the same rows as the reference query and the
semantically equivalent metamorphic_no_pushdown query.

## Actual Behavior

The optimized query returns only `{1}`, while the reference and no-pushdown
forms return `{1,3}`. `EXPLAIN` shows a `Run Condition` on the window
aggregate, indicating that the optimization changed query semantics.