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Subject: BUG #19633: Unexpected results of IN (subquery) with a
 non-deterministic collation
To: pgsql-bugs@lists.postgresql.org
From: PG Bug reporting form <noreply@postgresql.org>
Cc: syzhong16@gmail.com
Reply-To: syzhong16@gmail.com, pgsql-bugs@lists.postgresql.org
Date: Thu, 20 Aug 2026 03:49:53 +0000
Message-ID: <19633-647cd4c73a84b085@postgresql.org>
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 <https://www.postgresql.org/message-id/19633-647cd4c73a84b085%40postgresql.org>
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The following bug has been logged on the website:

Bug reference:      19633
Logged by:          Suyang Zhong
Email address:      syzhong16@gmail.com
PostgreSQL version: 19beta3
Operating system:   Ubuntu 22.04
Description:       =20

Hi,

Consider the following test case:

```
CREATE COLLATION ci (provider =3D icu, locale =3D 'und-u-ks-level1',
deterministic =3D false);

CREATE TABLE t_lhs(c1 text COLLATE ci);
CREATE TABLE t_rhs(c0 text);
INSERT INTO t_lhs VALUES ('a'), ('x'), ('y');
INSERT INTO t_rhs VALUES ('a'), ('a');
ANALYZE t_lhs;
ANALYZE t_rhs;
INSERT INTO t_rhs VALUES ('A');

SELECT c1, c1 IN (SELECT c0 FROM t_rhs) AS p FROM t_lhs;
-- a | t
-- x | f
-- y | f

SELECT count(*) FROM t_lhs WHERE c1 IN (SELECT c0 FROM t_rhs);
-- Expected: 1, Actual: 2
```

The predicate evaluates to true for one row, so filtering on the same
predicate should return one row.

Reproduced on 20devel and 19beta3.