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Subject: Re: Possible G2-item at SERIALIZABLE
From: Andrey Borodin <x4mmm@yandex-team.ru>
In-Reply-To: <165342c0-0c75-461e-b334-b997639ad48d@aphyr.com>
Date: Sun, 31 May 2026 18:19:55 +0500
Cc: PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
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Message-Id: <84AC98E5-3387-40A5-A258-C6E354349154@yandex-team.ru>
References: <165342c0-0c75-461e-b334-b997639ad48d@aphyr.com>
To: Kyle Kingsbury <aphyr@aphyr.com>
Archived-At: 
 <https://www.postgresql.org/message-id/84AC98E5-3387-40A5-A258-C6E354349154%40yandex-team.ru>
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Hello Kyle!

> On 22 May 2026, at 21:44, Kyle Kingsbury <aphyr@aphyr.com> wrote:
>=20
> I am relatively confident that isolation levels and transactions are
> being used correctly, because when I adjust the isolation level from
> Serializable to (e.g.) Read Committed, I see vastly different =
anomalies.
> I can also reproduce the problem both using JDBC's transaction calls,
> and with explicit BEGIN; SET TRANSACTION ISOLATION LEVEL
> SERIALIZABLE; ...; COMMIT;, which makes me think that it's not a bug =
in
> the way next.jdbc or the JDBC driver handle transactions.

I agree the multi-statement path looks correct, and I don't think =
next.jdbc or the
driver are at fault either. But I think there's a narrower issue that =
both of your
checks leave untouched: single-operation transactions are never wrapped =
in
a transaction at all.

In append's invoke!, a transaction is only opened when it has more than =
one op [0].

Could you confirm whether you still observe G2-item anomalies with a =
server
configured default_transaction_isolation =3D 'serializable' (so that the
single-statement operations are certainly Serializable)? If they persist =
under that
setting I'll dig further...


Best regards, Andrey Borodin.

[0] =
https://github.com/jepsen-io/sql/blob/6e34b76e2ac6a1c3edb5a9cab1c835eda4ca=
4c5e/src/jepsen/sql/append.clj#L155-L160=