Horton

Re: BUG #19406: substring(text) fails on valid UTF-8 toasted value in PostgreSQL 15.16

Noah Misch <noah@leadboat.com>

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From: Noah Misch <noah@leadboat.com>
To: ranvis@gmail.com, pgsql-bugs@lists.postgresql.org
Cc: thomas.munro@gmail.com
Date: 2026-02-13T17:27:02Z
Lists: pgsql-bugs 
On Fri, Feb 13, 2026 at 07:46:22AM +0000, PG Bug reporting form wrote:
> 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.

> user=> select substring(data from 1 for 1) from toast_repro;
> ERROR:  22021: invalid byte sequence for encoding "UTF8": 0xe6 0x97

Thanks for the report.  That is a bug and a regression; I regret missing it
during review.  The substring operation works by taking a 4-byte slice from
the toasted value (4 bytes being the max length of a UTF8 char in PostgreSQL),
the finding the actual first character within those bytes.  However, it
incorrectly requires those four bytes to be a valid UTF8 string.  I'll start
on a fix.

Commits

  1. Suppress new "may be used uninitialized" warning.

  2. Fix SUBSTRING() for toasted multibyte characters.

  3. pg_mblen_range, pg_mblen_with_len: Valgrind after encoding ereport.