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Subject: BUG #19406: substring(text) fails on valid UTF-8 toasted value in
 PostgreSQL 15.16
To: pgsql-bugs@lists.postgresql.org
From: PG Bug reporting form <noreply@postgresql.org>
Cc: ranvis@gmail.com
Reply-To: ranvis@gmail.com, pgsql-bugs@lists.postgresql.org
Date: Fri, 13 Feb 2026 07:46:22 +0000
Message-ID: <19406-9867fddddd724fca@postgresql.org>
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 <https://www.postgresql.org/message-id/19406-9867fddddd724fca%40postgresql.org>
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The following bug has been logged on the website:

Bug reference:      19406
Logged by:          SAT=C5=8C Kentar=C5=8D
Email address:      ranvis@gmail.com
PostgreSQL version: 15.16
Operating system:   Linux 5.14.0-611.27.1.el9_7.x86_64
Description:       =20

After upgrading from PostgreSQL 15.15 to 15.16, substring(text) raises:
>ERROR: invalid byte sequence for encoding "UTF8": 0xe6 0x97
on valid UTF-8 text stored in a TOAST-compressed column.

The same SQL works on PostgreSQL 15.15.
The data is valid UTF-8. The problem occurs only when operating on a
TOAST-compressed value.

Steps to Reproduce:
$ psql
psql (15.16)
Type "help" for help.

user=3D> \x
Expanded display is on.
user=3D> \set VERBOSITY verbose
user=3D> create table toast_repro (data text);
CREATE TABLE
user=3D> insert into toast_repro select repeat('=E6=97=A5=E5=90=91', 2000);=
 -- e6 97 a5 e5 90
91
INSERT 0 1
user=3D> select pg_column_size(data), octet_length(data) from toast_repro;
-[ RECORD 1 ]--+------
pg_column_size | 153
octet_length   | 12000
user=3D> select substring(data from 1 for 2) from toast_repro;
-[ RECORD 1 ]---
substring | =E6=97=A5=E5=90=91
user=3D> select substring(data from 1 for 1) from toast_repro;
ERROR:  22021: invalid byte sequence for encoding "UTF8": 0xe6 0x97
LOCATION:  report_invalid_encoding_int, mbutils.c:1796
user=3D> select substring('=E6=97=A5=E5=90=91' from 1 for 1);
-[ RECORD 1 ]-
substring | =E6=97=A5
user=3D> select substring(data::varchar(100) from 1 for 1) from toast_repro;
-[ RECORD 1 ]-
substring | =E6=97=A5
user=3D> select substring(data from 1 for 1) from ( select data || '' as da=
ta
from toast_repro) t;
-[ RECORD 1 ]-
substring | =E6=97=A5

Environment:
version | PostgreSQL 15.16 on x86_64-pc-linux-gnu, compiled by gcc (GCC)
11.5.0 20240719 (Red Hat 11.5.0-11), 64-bit
server_encoding | UTF8
client_encoding | UTF8
lc_collate | ja_JP.UTF-8
lc_ctype | ja_JP.UTF-8