Re: Is the "ACCESS EXCLUSIVE" lock for TRUNCATE really necessary?

"Florian G. Pflug" <fgp(at)phlo(dot)org> writes:
> Tom Lane wrote:
>> Until when? How would you synchronize the switchover?

> Every snapshot would either contain the old, or the new version of
> the corresponding pg_class tuple. The ones using the old version
> couldn't possible be writer, only reader (TRUNCATE would still need
> to acquire a lock that ensures that). New transactions started after
> the commit of the truncate would see the new version, and use
> the new datafile.

Wrong. *All* transactions read the system catalogs with SnapshotNow.

There's been some discussion of using MVCC semantics on catalog
accesses, but we are very far from being ready to do that; the
implications will reach into every part of the backend. To give just
one example of why this isn't necessarily a hot idea, suppose that
transaction A adds a constraint to a table T and commits, and then
transaction B (which started before A and hasn't previously touched T)
comes along and inserts a tuple into T that violates the constraint.
If B uses an old snapshot for looking up T's catalog info then it will
not see the constraint that it must satisfy. Locking does not help
since A released its locks on T before B would have tried to take any
lock.

regards, tom lane