Re: COPY with hints, rebirth

On Sat, Mar 3, 2012 at 1:14 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Fri, Mar 2, 2012 at 8:58 PM, Noah Misch <noah(at)leadboat(dot)com> wrote:
>> On Fri, Mar 02, 2012 at 08:46:45AM +0000, Simon Riggs wrote:
>>> On Thu, Mar 1, 2012 at 8:49 PM, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>>> > It's still broken:
>>
>> [BEGIN;TRUNCATE;SAVEPOINT;COPY;ROLLBACK TO]
>>
>>> So this approach isn't the one...
>>>
>>> The COPY FREEZE patch provides a way for the user to say explicitly
>>> that they don't really care about these MVCC corner cases and as a
>>> result allows us to avoid touching XidInMVCCSnapshot() at all. So
>>> there is still a patch on the table.
>>
>> You can salvage the optimization by tightening its prerequisite: use it when
>> the current subtransaction or a child thereof created or truncated the table.
>> A parent subtransaction having done so is acceptable for the WAL avoidance
>> optimization but not for this.
>
> I misread your earlier comment. Yes, that will make this work correctly.
>
>> Incidentally, I contend that we should write frozen tuples to new/truncated
>> tables unconditionally.  The current behavior of making old snapshots see the
>> table as empty violates atomicity at least as badly as letting those snapshots
>> see the future-snapshot contents.  But Marti has a sound proposal that would
>> interact with your efforts here to avoid violating atomicity at all:
>> http://archives.postgresql.org/message-id/CABRT9RBRMdsoz8KxgeHfb4LG-ev9u67-6DLqvoiibpkKhTLQfw@mail.gmail.com
>
> Thank you for bringing this to my attention.
>
> This will make this optimisation work correctly without adding
> anything to the main code path of XidInMVCCSnapshot() and without the
> annoying FREEZE syntax. So this puts the patch squarely back on the
> table.
>
> I'll do another version of this later today designed to work with the
> StrictMVCC patch.

OK, so latest version of patch handling the subtransaction issues
correctly. Thanks to Noah and Heikki for identifying them and hitting
me with the clue stick.

So this version of patch has negligible effect on mainline performance
though leaves newly loaded data visible in ways that would break MVCC.
So this patch relies on the safe_truncate.v2.patch posted on separate
thread.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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