Re: BUG #19519: REPACK can fail due to missing chunk for toast value

On Thu, Jul 9, 2026 at 11:13 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>
> On Thu, Jul 09, 2026 at 08:17:06AM +0300, Konstantin Knizhnik wrote:
> > Changing `table_relation_fetch_toast_slice` signature breaks compatibility
> > with some extensions, for example duckdb.
> > I wonder if we can change only `bool (*relation_fetch_toast_slice)(...,
> > bool missing_ok)` callback signature, but preserve signature of
> > `table_relation_fetch_toast_slice`, adding one more function
> > `table_relation_try_fetch_toast_slice`:
>
> Thanks for the input.
>
> Adding more functions implies to change the size of TableAmRoutine,
> which may be bad. For now I'd be tempted to just focus on how to fix
> the problem on HEAD in a way we are happy with. We could sort out an
> optional back-branch version as a second step.

I was reviewing this patch, haven't yet completed the review but
noticed one discrepancy with the patch

Prior to this patch, if att->attstorage == TYPSTORAGE_PLAIN,
toast_tuple_init() called detoast_attr(), which fetches and
decompresses if VARATT_IS_EXTERNAL_ONDISK and VARATT_IS_COMPRESSED.
Now with patch when (flags & TOAST_MISSING_OK) is set,
detoast_external_attr_extended() fetches raw bytes from pg_toast if
(VARATT_IS_EXTERNAL_ONDISK(attr)), leaving compressed external datums
compressed. So this seems to be a change in behavior for external
compressed attributes when the attribute storage is set to PLAIN. Am
I missing something?

--
Regards,
Dilip Kumar
Google