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From: Thomas Munro <thomas.munro@gmail.com>
Date: Sun, 15 Feb 2026 09:10:00 +1300
Message-ID: <CA+hUKGKJm6Zi0P5vp=ZnhS=WN0_13tUX2aS5EzQQu3Ci=xhVew@mail.gmail.com>
Subject: Re: BUG #19406: substring(text) fails on valid UTF-8 toasted value in
 PostgreSQL 15.16
To: Noah Misch <noah@leadboat.com>
Cc: ranvis@gmail.com, pgsql-bugs@lists.postgresql.org
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On Sun, Feb 15, 2026 at 8:33=E2=80=AFAM Noah Misch <noah@leadboat.com> wrot=
e:
> - 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% reduc=
tion,
> but I didn't look too closely.  I guessed such a change would be less cle=
arly
> 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.