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Subject: BUG #19641: Unexpected results on an SP-GiST indexed column 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: Fri, 28 Aug 2026 04:10:09 +0000
Message-ID: <19641-fbd0937eac4bf27e@postgresql.org>
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The following bug has been logged on the website:

Bug reference:      19641
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 nd (provider =3D icu, locale =3D 'und-u-ks-level2',
deterministic =3D false);

CREATE TABLE t0(c1 text);
INSERT INTO t0 VALUES ('ALPHA');
CREATE INDEX ON t0 USING spgist (c1 COLLATE nd);

SELECT c1, c1 COLLATE nd =3D 'alpha' AS p FROM t0;
-- ALPHA | t

SET enable_seqscan =3D off;
SELECT count(*) FROM t0 WHERE c1 COLLATE nd =3D 'alpha';
-- Expected: 1, Actual: 0

RESET enable_seqscan;
SELECT count(*) FROM t0 WHERE c1 COLLATE nd =3D 'alpha';
-- 1

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

The original test case, where the planner chooses the index by itself:

CREATE TABLE t1(c1 text);
INSERT INTO t1
SELECT CASE WHEN i % 3 =3D 0 THEN 'alpha'
            WHEN i % 3 =3D 1 THEN 'ALPHA'
            ELSE 'beta' END
FROM generate_series(1, 340) AS i;
CREATE INDEX ON t1 USING spgist (c1 COLLATE nd);

SELECT count(*) FROM t1 WHERE c1 COLLATE nd =3D 'alpha';
-- Expected: 227, Actual: 113

With the v2 patch from #19633 applied, this case is still unchanged.
Reproduced on 20devel, 19beta3 and 18.6.