From: Tom Lane <tgl@sss.pgh.pa.us>
To: Laurenz Albe <laurenz.albe@cybertec.at>
Cc: Andres Freund <andres@anarazel.de>
Cc: suchithjn22@gmail.com
Cc: pgsql-docs@lists.postgresql.org
Subject: Re: The documentation for storage type 'plain' actually allows single byte header
Date: Mon, 16 Jan 2023 11:50:11 -0500
Message-ID: <1887216.1673887811@sss.pgh.pa.us> (raw)
In-Reply-To: <245d0f085c68fe0b877235d76a7c32a71454f39b.camel@cybertec.at>
References: <167336599095.2667301.15497893107226841625@wrigleys.postgresql.org>
<d99b3e0beebdde0741d28035aaea7f4af56fa050.camel@cybertec.at>
<1698750.1673818827@sss.pgh.pa.us>
<245d0f085c68fe0b877235d76a7c32a71454f39b.camel@cybertec.at>
Laurenz Albe <laurenz.albe@cybertec.at> writes:
> On Sun, 2023-01-15 at 16:40 -0500, Tom Lane wrote:
>> 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?
The original motivation for that whole mechanism was to protect data
types for which the C functions haven't been upgraded to support
non-traditional varlena headers. So I was worried that this behavior
would somehow break those cases (which still exist, eg oidvector and
int2vector). However, the thing that actually marks such a datatype
is that pg_type.typstorage is PLAIN, and as far as I can find we do
still honor that case in full. If that's the case then every tupdesc
we ever create for such a column will say PLAIN, so there's no
opportunity for the wrong thing to happen.
So maybe it's okay to move the goalposts and acknowledge that setting
attstorage to PLAIN isn't a complete block on applying toast-related
transformations. I wonder though whether short-header is the only
case that can slide through. In particular, for "INSERT ... SELECT
FROM othertable", I suspect it's possible for a compressed-in-line
datum to slide through without decompression. (We certainly must
fix out-of-line datums, but that doesn't necessarily mean we undo
compression.) So I'm not convinced that the proposed wording is
fully correct yet.
regards, tom lane
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To: pgsql-docs@postgresql.org
Cc: tgl@sss.pgh.pa.us, laurenz.albe@cybertec.at, andres@anarazel.de, suchithjn22@gmail.com, pgsql-docs@lists.postgresql.org
Subject: Re: The documentation for storage type 'plain' actually allows single byte header
In-Reply-To: <1887216.1673887811@sss.pgh.pa.us>
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