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

On Fri, Feb 13, 2026 at 02:48:04PM -0800, Noah Misch wrote:
> On Fri, Feb 13, 2026 at 09:27:02AM -0800, Noah Misch 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.
> 
> Attached.  I may add some more tests, e.g. a toasted invalid string where the
> detoasted length is less than the slice we request.

Tests already covered that in particular, but I added some other tests.  I
think this is now ready.  Review welcome.  I have a Valgrind test run ongoing.