On Wed, Mar 7, 2012 at 7:49 AM, Noah Misch <n...@leadboat.com> wrote: > On Tue, Mar 06, 2012 at 08:36:05AM -0500, Robert Haas wrote: >> On Tue, Mar 6, 2012 at 5:43 AM, Noah Misch <n...@leadboat.com> wrote: >> >> Well, consider something like CLUSTER. ?It's perfectly OK for CLUSTER >> >> to operate on a table that has been truncated since CLUSTER's snapshot >> >> was taken, and no serialization anomaly is created that would not have >> >> already existed as a result of the non-MVCC-safe TRUNCATE. ?On the >> >> other hand, if CLUSTER operates on a table that was created since >> >> CLUSTER's snapshot was taken, then you have a bona fide serialization >> >> anomaly. >> > >> > Core CLUSTER does not use any MVCC snapshot. ?We do push one for the >> > benefit >> > of functions called during the reindex phase, but it does not appear that >> > you >> > speak of that snapshot. ?Could you elaborate this example? >> >> Imagine this: >> >> - Transaction #1 acquires a snapshot. >> - Transaction #2 creates tables A, inserts a row into table B, and then >> commits. >> - Transaction #1 tries to CLUSTER A and then select from B. >> >> The only serial execution schedules are T1 < T2, in which case the >> transaction fails, or T2 < T1, in which case the row is seen. But >> what actually happens is that the row isn't seen and yet the >> transaction doesn't fail. > > For the purpose of contemplating this anomaly, one could just as well replace > CLUSTER with GRANT, COMMENT ON TABLE, or any other command that operates on a > table, correct? > > I agree this test case is good to keep in mind while designing, but we could > well conclude not to bother improving it.
All true. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers