Horton

Re: Potential G2-item cycles under serializable isolation

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Kyle Kingsbury <aphyr@jepsen.io>
Cc: Thomas Munro <thomas.munro@gmail.com>, Peter Geoghegan <pg@bowt.ie>, Kevin Grittner <kgrittn@gmail.com>, Jeff Davis <pgsql@j-davis.com>, PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Date: 2020-06-11T19:54:36Z
Lists: pgsql-bugs 
Hi,

On 2020-06-11 08:39:13 -0400, Kyle Kingsbury wrote:
> On 6/11/20 1:40 AM, Thomas Munro wrote:
> > What do you think about this documentation update?
> 
> I think this is really helpful! You can go a little further if you like.
> Right now, SSI vs serializable and SI vs RR are both described as
> "differences in behavior", which kinda leaves it unclear as to how those
> levels are related. If you want to follow Berenson, Adya, et al.'s broad
> interpretation, I'd say something like
> 
> "Snapshot isolation prevents some anomalies, like phantoms, which repeatable
> read allows. It also allows some anomalies, like G2-item (write skew), which
> repeatable read prevents."

I think the percentage of readers that understand "G2-item" or even
"write skew", but don't already know what snapshot isolation implies, is
pretty close to zero. I like the idea of having a bit more detail, but I
think it'd have to be worded differently to be beneficial.

I'd also phrase it as "some definitions of repeatable read", but ... :).

Intuitively, and that's not meant as an argument for anything, I find
prohibiting write skew in something named "repeatable read" just plain
odd.  Leaving naming aside, I think there's good practical reasons for
applications using read-committed, snapshot isolation, and
serializable. It's not obvious to me that there are cases where the
Berenson definition of RR is something that's desirable to use over
RR/SI/S.


I suspect what we really want, medium term, is some guidelines around
what isolation levels are appropriate for which use-case. But that's a
nontrivial thing to write, I think, and there's lot of room for
disagreements...


Greetings,

Andres Freund

Commits

  1. Doc: Add references for SI and SSI.

  2. Improve comments for [Heap]CheckForSerializableConflictOut().

  3. Avoid update conflict out serialization anomalies.

  4. Implement genuine serializable isolation level.