On Mon, Dec 23, 2013 at 6:53 AM, Andres Freund <and...@2ndquadrant.com> wrote: > On 2013-12-22 20:45:02 -0500, Robert Haas wrote: >> I suspect we ought to extend this to rewriting variants of ALTER TABLE >> as well, but a little thought is needed there. ATRewriteTables() >> appears to just call heap_insert() for each updated row, which if I'm >> not mistaken is an MVCC violation - offhand, it seems like a >> transaction with an older MVCC snapshot could access the table for >> this first time after the rewriter commits, and fail to see rows which >> would have appeared to be there before the rewrite. (I haven't >> actually tested this, so watch me be wrong.) If we're OK with an MVCC >> violation here, we could just pass HEAP_INSERT_FROZEN and have a >> slightly different flavor of violation; if not, this needs some kind >> of more extensive surgery quite apart from what we do about freezing. > > Yes, rewriting ALTER TABLEs certainly aren't MVCC safe. I thought that > was documented, but apparently not. > I am not sure it can be fixed easily using the tricks CLUSTER plays, > there might be nasty edge-cases because of the changes in the column > definition. Certainly not a trivial project. > > I think we should leave ALTER TABLE as a completely separate project and > just improve CLUSTER for now. The practical impact of rewriting ALTER > TABLEs not freezing is far smaller, because they are very seldomly > performed in bigger databases.
OK, I have committed my patch to make CLUSTER and VACUUM FULL freeze aggressively, and have left ALTER TABLE alone for now. It would be nice to get to the point where a database-wide VACUUM FULL would serve to bump datfrozenxid, so as to avoid having to give this sort of advice: http://www.postgresql.org/message-id/CA+Tgmobth=aqkwmwtcsqlaenv59gt4g3oqpqs45cb+fvg9m...@mail.gmail.com However, it doesn't, quite: a bare VACUUM FULL now bumps relfrozenxid for every table in the database *except pg_class*. It does call vac_update_datfrozenxid() afterwards, but that only helps if pg_class is not among the things holding back datfrozenxid. I haven't dug into this much yet, but I think it might be worth fixing. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
