Re: The documentation for storage type 'plain' actually allows single byte header

On Fri, 2023-09-29 at 18:19 -0400, Bruce Momjian wrote:
> On Thu, Jan 12, 2023 at 03:43:57PM +0100, Laurenz Albe wrote:
> > On Tue, 2023-01-10 at 15:53 +0000, PG Doc comments form wrote:
> > > https://www.postgresql.org/docs/devel/storage-toast.html - This is the
> > > development version.
> > >
> > > > PLAIN prevents either compression or out-of-line storage; furthermore it
> > > > disables use of single-byte headers for varlena types. This is the only
> > > > possible strategy for columns of non-TOAST-able data types.
> > >
> > > However, it does allow "single byte" headers. How to verify this?
> > >
> > > CREATE EXTENSION pageinspect;
> > > CREATE TABLE test(a VARCHAR(10000) STORAGE PLAIN);
> > > INSERT INTO test VALUES (repeat('A',10));
> > >
> > > Now peek into the page with pageinspect functions
> > >
> > > SELECT left(encode(t_data, 'hex'), 40) FROM
> > > heap_page_items(get_raw_page('test', 0));
> > >
> > > This returned value of "1741414141414141414141".
> > > Here the first byte 0x17 = 0001 0111 in binary.
> > > Length + 1 is stored in the length bits (1-7). So Len = 0001011-1 = (11-1)
> > > [base-10] = 10 [base-10]
> > > which exactly matches the expected length. Further the data "41" repeated 10
> > > times also indicates character A (65 or 0x41 in ASCII) repeated 10 times.
> > >
> > > So....This does **not** disable 1-B header. That sentence should be removed
> > > from the documentation unless this is a bug.
> >
> > I think that the documentation is wrong.  The attached patch removes the
> > offending half-sentence.
> >
> > Yours,
> > Laurenz Albe
>
> > From 5bf0b43fe73384a21f59d9ad1f7a8d7cbc81f8c4 Mon Sep 17 00:00:00 2001
> > From: Laurenz Albe <laurenz.albe@cybertec.at>
> > Date: Thu, 12 Jan 2023 15:41:56 +0100
> > Subject: [PATCH] Fix documentation for STORAGE PLAIN
> >
> > Commit 3e23b68dac0, which introduced single-byte varlena headers,
> > added documentation that STORAGE PLAIN would prevent such single-byte
> > headers.  This has never been true.
> > ---
> >  doc/src/sgml/storage.sgml | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/doc/src/sgml/storage.sgml b/doc/src/sgml/storage.sgml
> > index e5b9f3f1ff..4795a485d0 100644
> > --- a/doc/src/sgml/storage.sgml
> > +++ b/doc/src/sgml/storage.sgml
> > @@ -456,9 +456,7 @@ for storing <acronym>TOAST</acronym>-able columns on disk:
> >      <listitem>
> >       <para>
> >        <literal>PLAIN</literal> prevents either compression or
> > -      out-of-line storage; furthermore it disables use of single-byte headers
> > -      for varlena types.
> > -      This is the only possible strategy for
> > +      out-of-line storage.  This is the only possible strategy for
> >        columns of non-<acronym>TOAST</acronym>-able data types.
> >       </para>
> >      </listitem>
> > --
> > 2.39.0
> >
>
> Where did we end with this?  Is a doc patch the solution?

I don't think this went anywhere, and a doc patch is not the solution.

Tom has argued convincingly that single-byte headers are an effect of the TOAST
system, and that STORAGE PLAIN should disable all effects of TOAST.

So this would need a code patch.

Yours,
Laurenz Albe