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

On Fri, Feb 10, 2012 at 6:42 AM, Noah Misch <noah(at)leadboat(dot)com> wrote:
> I like the design you have chosen.  It would find applications beyond
> TRUNCATE, so your use of non-specific naming is sound.  For example, older
> snapshots will see an empty table "t" after "CREATE TABLE t AS SELECT 1"
> commits; that's a comparable MVCC anomaly.  Some of our non-MVCC-safe commands
> should perhaps just become MVCC-safe, but there will always be use cases for
> operations that shortcut MVCC.  When one truly does want that, your proposal
> for keeping behavior consistent makes plenty of sense.

I guess I'm not particularly excited by the idea of trying to make
TRUNCATE MVCC-safe. I notice that the example involves the REPEATABLE
READ isolation level, which is already known to be busted in a variety
of ways; that's why we now have SERIALIZABLE, and why most people use
READ COMMITTED. Are there examples of this behavior at other
isolation levels?

But I have to admit I'm intrigued by the idea of extending this to
other cases, if there's a reasonable way to do that. For example, if
we could fix things up so that we don't see a table at all if it was
created after we took our snapshot, that would remove one of the
obstacles to marking pages bulk-loaded into that table with FrozenXID
and PD_ALL_VISIBLE from the get-go. I think a lot of people would be
mighty happy about that.

But the necessary semantics seem somewhat different. For TRUNCATE,
you want to throw a serialization error; but is that also what you
want for CREATE TABLE? Or do you just want it to appear empty?

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company