Message-ID: <dbebe820-eec3-d643-aab7-26a33392c27b@xs4all.nl>
Date: Tue, 29 Mar 2022 13:29:15 +0200
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Subject: Re: TRAP: FailedAssertion("HaveRegisteredOrActiveSnapshot()", File:
 "toast_internals.c", Line: 670, PID: 19403)
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To: Matthias van de Meent <boekewurm+postgres@gmail.com>,
 Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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From: Erik Rijkers <er@xs4all.nl>
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Op 29-03-2022 om 12:50 schreef Matthias van de Meent:
> On Tue, 29 Mar 2022 at 11:10, Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote:
>>
>> At Tue, 29 Mar 2022 17:06:21 +0900 (JST), Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote in
>>> At Mon, 28 Mar 2022 18:36:46 +0900 (JST), Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote in
>>> Then, I found that portal->holdSnapshot is that.  I came up with the
>>> attached.  It does the follows:
>>>
>>> 1. Teach PlannedStmtRequiresSnapshot() to return true for FetchStmt.
>>>
>>> 2. Use holdSnapshot in RunFromStore if any.
>>>
>>>
>>> The rerpducer is reduced to as small as the following.
>>>
>>> CREATE TABLE t (a text);
>>> INSERT INTO t VALUES('some random text');
>>> BEGIN;
>>> DECLARE c CURSOR FOR SELECT * FROM t;
>>> FETCH ALL IN c;
>>>
>>> But I haven't come up with a reasonable way to generate the 'some
>>> random text' yet.
>>
>> I gave up and took a straightforward way to generate one.
>>
>> I don't like that it uses a fixed length for the random text, but
>> anyway it works for now...
> 
> An shorter (?) reproducer might be the following, which forces any
> value for 'a' to be toasted and thus triggering the check in
> init_toast_snapshot regardless of value length:
> 
> CREATE TABLE t (a text);
> ALTER TABLE t ALTER COLUMN a SET STORAGE EXTERNAL;
> INSERT INTO t VALUES ('toast');
> BEGIN;
> DECLARE c CURSOR FOR SELECT * FROM t;
> FETCH ALL IN c;

Excellent. That indeed immediately forces the error.

(and the patch prevents it)

Thanks!


> 
> Enjoy,
> 
> -Matthias