Re: RFC: Making TRUNCATE more "MVCC-safe"

On Tue, Mar 06, 2012 at 08:36:05AM -0500, Robert Haas wrote:
> On Tue, Mar 6, 2012 at 5:43 AM, Noah Misch <noah@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.