Re: Dead code with short varlenas in toast_save_datum()

Michael Paquier <michael@paquier.xyz>

From: Michael Paquier <michael@paquier.xyz>

To: Nikhil Kumar Veldanda <veldanda.nikhilkumar17@gmail.com>

Cc: Postgres hackers <pgsql-hackers@lists.postgresql.org>

Date: 2025-08-07T01:01:26Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Add isolation test for TOAST value reuse during CLUSTER
- f57e214d1cbb 19 (unreleased) landed
Add SQL test for TOAST value allocations on rewrite
- 69f75d671475 19 (unreleased) landed
Add regression test for short varlenas saved in TOAST relations
- 225ebfe30a1a 19 (unreleased) landed

On Wed, Aug 06, 2025 at 10:30:54AM +0900, Michael Paquier wrote:
> Ah, thanks, nice one.  I did not consider the trick of using two
> attributes to bypass the check when externalizing the tuple.  I'll go
> add a test in strings.sql among these lines, with some TOAST slice
> scans based on substr().

Done that as of 225ebfe30a1a, with some additions:
- A check on pg_column_compression(), to make sure that the short
varlena path leads to an uncompressed on-disk Datum.
- Some substr() for the values, to cross-check slice reads.
- A query based on the TOAST relation's chunk_seq, making sure that we
have taken toast_save_datum() two times when inserting a tuple with
the two toastable attributes.

The buildfarm is OK with it, as is the latest patch for the 8-byte
TOAST values posted here:
https://www.postgresql.org/message-id/aIyCz4T858Kcm4UU@paquier.xyz

The patch posted there needs a rebase, following the changes in
varatt.h done in e035863c9a04.  Will post a rebase very soon.
--
Michael