No particular effort; I saw a talk on the topic. I said it's academic because I can't think of any real world example where this would matter (the other definition of the word "academic").
On Thu, Sep 19, 2013 at 12:44 AM, Kevin Grittner <kgri...@ymail.com> wrote: > Aaron Carlisle <aaron.carli...@gmail.com> wrote: > > > The documentation states that "concurrent execution of a set of > > Serializable transactions is guaranteed to produce the same > > effect as running them one at a time in some order." > > > > I'm not sure how the following behavior fits that definition. > > (Note that this is just an experiment, not a use case. Purely > > academic.) I run these transactions sequentially, and I get a > > different result than if I run them concurrently. > > > It seems like one of these should error out and not commit, so I > > must be missing some stipulation. > > > [ write skew in pg_class with table creation and check for table > > existence in two concurrent queries ] > > You are correct, and not missing anything -- when serializable > behavior was added (and through 9.3) the system catalogs were not > fully transactional using the MVCC semantics. I'm afraid there > were many strange things that could happen with the system > catalogs, including having an object which was being updated be > missed or seen twice by a concurrent transaction, none of which was > documented. Since the serializable techniques in PostgreSQL are > based on the MVCC snapshot isolation techniques, it was not > possible to cover DDL in the serializable implementation using > Serializable Snapshot Isolation. > > I assume you can't create such problems in a stable schema; but > only when serializable transactions include DDL? > > On the 9.4 development branch access to the system catalogs has > recently been made to use MVCC techniques, so it may be possible to > extend serializable behavior to the catalogs in 9.4 or later. > > I don't suppose you want to help develop a patch for that? :-) > > Out of curiosity, is there a particular academic effort this > question came out of that you can talk about? > > -- > Kevin Grittner > EDB: http://www.enterprisedb.com > The Enterprise PostgreSQL Company >
