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

On Fri, Mar 9, 2012 at 2:59 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Fri, Mar 9, 2012 at 3:46 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Wed, Mar 7, 2012 at 5:41 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>>>> Case #2 is certainly a problem for FrozenXID as well, because anything
>>>> that's marked with FrozenXID is going to look visible to everybody,
>>>> including our older snapshots.  And I gather you're saying it's also a
>>>> problem for HEAP_XMIN_COMMITTED.
>>>
>>> The problem there is that later subtransactions often have xids that
>>> are greater than xmax, so the xid shows as running when we do
>>> XidInMVCCSnapshot(), which must then be altered for this one weird
>>> case. I tried that and not happy with result.
>>
>> Altering XidInMVCCSnapshot() seems like a good thing to avoid, but I
>> confess I don't quite follow what you're describing here otherwise.
>>
>>>> I had assumed that the way we were
>>>> fixing this problem was to disable these optimizations for
>>>> transactions that had more than one snapshot floating around.  I'm not
>>>> sure whether the patch does that or not, but I think it probably needs
>>>> to
>>>
>>> It does. I thought you already read the patch?
>>
>> I glanced over it, but did not look through it in detail.  I'll do a
>> more careful look at your next version.
>
> I'm not confident about the restrictions this patch imposes and we
> aren't close enough to a final version for me to honestly request this
> be considered for this release. I think its time to close this door
> for now.

OK, makes sense. I was trying to hold my nose, because I really would
like to see this stuff work better than it does, but I had my doubts,
too.

--
Robert Haas
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