On 11/09/2012 09:34 AM, Simon Riggs wrote:
On 8 November 2012 23:20, Hannu Krosing <ha...@2ndquadrant.com> wrote:
On 11/08/2012 08:51 PM, Simon Riggs wrote:
On 8 November 2012 17:07, Robert Haas <robertmh...@gmail.com> wrote:
On Wed, Nov 7, 2012 at 10:34 AM, Simon Riggs <si...@2ndquadrant.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 ?
We could... but that would require keeping a history of relfilenodes
for an object to allow for more than one TRUNCATE event.
MVCC does keep history of old relfilenodes.
I thought we were able to read old MVCC versions in pg_class for
this if the snapshot required data matching older pg_class record.
Tracking all
of that would be hard and I don't personally think its worth that
effort.
and we unlink the base file(s) only once nobody has a snapshot the can see
it ?
DELETE does that if that's the semantics you want.
or are there some subtler problems (I was under impression that we already
did this as described above) ?
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