Message-ID: <245d0f085c68fe0b877235d76a7c32a71454f39b.camel@cybertec.at>
Subject: Re: The documentation for storage type 'plain' actually allows
 single byte header
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andres Freund <andres@anarazel.de>, suchithjn22@gmail.com,
	pgsql-docs@lists.postgresql.org
Date: Mon, 16 Jan 2023 14:07:48 +0100
In-Reply-To: <1698750.1673818827@sss.pgh.pa.us>
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	 <d99b3e0beebdde0741d28035aaea7f4af56fa050.camel@cybertec.at>
	 <1698750.1673818827@sss.pgh.pa.us>
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On Sun, 2023-01-15 at 16:40 -0500, Tom Lane wrote:
> Laurenz Albe <laurenz.albe@cybertec.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; furthermo=
re it
> > > > disables use of single-byte headers for varlena types. This is the =
only
> > > > possible strategy for columns of non-TOAST-able data types.
>=20
> > > 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));
> > >=20
> > > Now peek into the page with pageinspect functions
> > >=20
> > > SELECT left(encode(t_data, 'hex'), 40) FROM
> > > heap_page_items(get_raw_page('test', 0));
> > >=20
> > > This returned value of "1741414141414141414141".
>=20
> > I think that the documentation is wrong.=C2=A0 The attached patch remov=
es the
> > offending half-sentence.
>=20
> The documentation is correct, what is broken is the code.

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

Yours,
Laurenz Albe