Aaron Carlisle <aaron.carlisle@xxxxxxxxx> 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 -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
