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

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.

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