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

On Sun, 2023-01-15 at 16:40 -0500, Tom Lane wrote:
> Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> writes:
> > On Tue, 2023-01-10 at 15:53 +0000, PG Doc comments form wrote:
> > > > 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".
>
> > I think that the documentation is wrong.  The attached patch removes the
> > offending half-sentence.
>
> The documentation is correct, what is broken is the code.

I see. But what is the reason for that anyway? Why not allow short varlena
headers if TOAST storage is set to PLAIN?

Yours,
Laurenz Albe