Horton

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

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Noah Misch <noah@leadboat.com>
Cc: ranvis@gmail.com, pgsql-bugs@lists.postgresql.org
Date: 2026-02-13T21:58:50Z
Lists: pgsql-bugs 
On Sat, Feb 14, 2026 at 6:27 AM Noah Misch <noah@leadboat.com> wrote:
> 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.

Ack.  Also looking into this.

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.