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Subject: BUG #19588: Semantically equivalent DISTINCT ON query returns
 different result when wrapped in MATERIALIZED CTE.
To: pgsql-bugs@lists.postgresql.org
From: PG Bug reporting form <noreply@postgresql.org>
Cc: dllggyx@outlook.com
Reply-To: dllggyx@outlook.com, pgsql-bugs@lists.postgresql.org
Date: Thu, 30 Jul 2026 09:56:45 +0000
Message-ID: <19588-e32b82433b3660ea@postgresql.org>
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 <https://www.postgresql.org/message-id/19588-e32b82433b3660ea%40postgresql.org>
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The following bug has been logged on the website:

Bug reference:      19588
Logged by:          Yuxiao Guo
Email address:      dllggyx@outlook.com
PostgreSQL version: 18.4
Operating system:   Ubuntu 20.04 x86-64, docker image postgres:18.4
Description:       =20

The two forms are semantically equivalent, but I observed different results.
The first version returned 1.0, while the second version returned empty set.

PoC:

CREATE TABLE t0(c0 numeric);
INSERT INTO t0 VALUES (1.0), (1.00);

-- Query A, result: {1.0}
SELECT * FROM (
  SELECT DISTINCT ON (c0) c0
  FROM t0
  ORDER BY c0, scale(c0) DESC
) t1
WHERE scale(c0) =3D 1;

-- Query B, result: empty set
WITH t1 AS MATERIALIZED (
  SELECT DISTINCT ON (c0) c0
  FROM t0
  ORDER BY c0, scale(c0) DESC
)
SELECT * FROM t1 WHERE scale(c0) =3D 1;