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: 2026-02-14T20:10:00Z
Lists: pgsql-bugs 
On Sun, Feb 15, 2026 at 8:33 AM Noah Misch <noah@leadboat.com> wrote:
> - slice_len is the amount *returned* from the toaster.  It's nonnegative.

Ah, right, that makes more sense.

> > The outline I had come up with before seeing your patch was: let's
> > just delete it.  The position search can check bounds incrementally,
> > following our general approach.  This avoids the reported problem by
> > ditching the pre-flight scan through the slice (up to 4x more
> > pg_mblen_XXX calls and memory access than we strictly need), and also
> > the special cases for empty strings since they already fall out of the
> > general behaviour (am I missing something?), not leaving much code
> > behind.
>
> Like you, I made a note that it's wasteful to make two mblen passes over the
> string.  I'm only seeing a 50% reduction in mblen calls, not an 80% reduction,
> but I didn't look too closely.  I guessed such a change would be less clearly
> correct, so I figured it would be less suitable for back branches.  Hence, I
> didn't draft it.

I was comparing to unpatched master, but yeah of course your patch
already gets part of the way there.

> My first impression, hurried due to the commit ETA in 30 minutes, is that this
> is less conservative and should hold for master-only.

Got it.  Will add it to the pile of master-only fallout from this area.

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.