Re: TRUNCATE SERIALIZABLE and frozen COPY

On 11/08/2012 08:51 PM, Simon Riggs wrote:
> On 8 November 2012 17:07, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Wed, Nov 7, 2012 at 10:34 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>>> For 9.2 we discussed having COPY setting tuples as frozen. Various
>>> details apply.
>>> Earlier threads:
>>> "RFC: Making TRUNCATE more "MVCC-safe"
>>> "COPY with hints, rebirth"
>>>
>>> I was unhappy with changing the behaviour of TRUNCATE, and still am.
>>> So the proposal here is to have a specific modifier on TRUNCATE
>>> command that makes it MVCC safe by throwing a serialization error.
>> I don't think I understand the proposal. Under what circumstances
>> would it throw a serialization error?
> If you ask for TRUNCATE SERIALIZABLE then if someone asks for data in
> the table and has a snapshot that can see earlier data then it will
> throw a serializable error. So its a new kind of TRUNCATE that is MVCC
> safe.
Can't we make it so that the reader with earlier snapshot sees the data
from the pre-truncation file ?

and we unlink the base file(s) only once nobody has a snapshot the can
see it ?

or are there some subtler problems (I was under impression that we
already did this as described above) ?

----------------
Hannu