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.a...@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
