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

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.

--
Robert Haas
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