Re: pg_column_toast_chunk_id: a function to get a chunk ID of a TOASTed value

On Fri, 8 Mar 2024 16:17:58 -0600
Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote:

> On Fri, Mar 08, 2024 at 03:31:55PM +0900, Yugo NAGATA wrote:
> > On Thu, 7 Mar 2024 16:56:17 -0600
> > Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote:
> >> to me. Do you think it's worth adding something like a
> >> pg_column_toast_num_chunks() function that returns the number of chunks for
> >> the TOASTed value, too?
> >
> > If we want to know the number of chunks of a specified chunk_id,
> > we can get this by the following query.
> >
> > postgres=# SELECT id, (SELECT count(*) FROM pg_toast.pg_toast_16384 WHERE chunk_id = id)
> > FROM (SELECT pg_column_toast_chunk_id(v) AS id FROM t);
>
> Good point. Overall, I think this patch is in decent shape, so I'll aim to
> commit it sometime next week.

Thank you.

>
> > +{ oid => '8393', descr => 'chunk ID of on-disk TOASTed value',
> > + proname => 'pg_column_toast_chunk_id', provolatile => 's', prorettype => 'oid',
> > + proargtypes => 'any', prosrc => 'pg_column_toast_chunk_id' },
>
> Note to self: this change requires a catversion bump.
>
> > +INSERT INTO test_chunk_id(v1,v2)
> > + VALUES (repeat('x', 1), repeat('x', 2048));
>
> Is this guaranteed to be TOASTed for all possible page sizes?

Should we use block_size?

SHOW block_size \gset
INSERT INTO test_chunk_id(v1,v2)
VALUES (repeat('x', 1), repeat('x', (:block_size / 4)));

I think this will work in various page sizes.
I've attached a patch in which the test is updated.

Regards,
Yugo Nagata

>
> --
> Nathan Bossart
> Amazon Web Services: https://aws.amazon.com

--
Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp>