Date: Tue, 11 Aug 2020 11:39:40 -0700
From: Andres Freund <andres@anarazel.de>
To: Pavel Biryukov <79166341370@yandex.ru>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>,
	PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Subject: Re: posgres 12 bug (partitioned table)
Message-ID: <20200811183940.z6hvnu4uptkhugom@alap3.anarazel.de>
References: <141051591267657@mail.yandex.ru>
 <1251598.1591289850@sss.pgh.pa.us>
 <CA+TgmoaN0kQa3nxToHtD8nRymSnUYaj4XL3zKq8Q_0AUJcQNPQ@mail.gmail.com>
 <3398357.1597160032@sss.pgh.pa.us>
 <1058041597161586@mail.yandex.ru>
 <20200811172241.izeyvksgevbtvhcd@alap3.anarazel.de>
 <24981597170121@mail.yandex.ru>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <24981597170121@mail.yandex.ru>
Precedence: bulk

Hi,

On 2020-08-11 21:31:52 +0300, Pavel Biryukov wrote:
> Entity Framework is an ORM for .Net (and .Net Core). It has different providers
> for different databases (NpgSql for Postgres). It uses Optimistic concurrency.
> The common use case is to use xmin as "concurrency token".
>  
> In code we make "var e = new Entity();", "dbContext.Add(e)" and
> "dbContext.SaveChanges()" (smth like that), and EF Core constructs sql for us,
> classical ORM;
>  
> When new row is inserted, EF makes an insert with "RETURNING xmin" to keep it
> as concurrency token for further updates (update is made like "where id = [id]
> AND xmin=[xmin]" to be sure the row hasn't been updated by other clients).

That's not really a safe use of xmin, e.g. it could have wrapped around
leading you to not notice a concurrent modification. Nor does it
properly deal with multiple statements within a transaction.  Perhaps
those are low enough risk for you, but I don't xmin is a decent building
block for this kind of thing.

Greetings,

Andres Freund