Re: ALTER TABLE lock strength reduction patch is unsafe

On Mon, Jan 2, 2012 at 6:41 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Mon, Jan 2, 2012 at 6:06 PM, Noah Misch <noah(at)leadboat(dot)com> wrote:
>> On Mon, Jan 02, 2012 at 05:09:16PM +0000, Simon Riggs wrote:
>>> Attached patch makes SnapshotNow into an MVCC snapshot, initialised at
>>> the start of each scan iff SnapshotNow is passed as the scan's
>>> snapshot. It's fairly brief but seems to do the trick.
>>
>> That's a neat trick.  However, if you start a new SnapshotNow scan while one is
>> ongoing, the primordial scan's snapshot will change mid-stream.
>
> Do we ever do that? (and if so, Why?!? or perhaps just Where?)

Just for the record, yes we do run multiple catalog scans in some
parts of the code.

So I can see how we might trigger 4 nested scans, using cache
replacement while scanning, so best assume more, with no guarantee of
them being neatly stacked for pop/push type access.

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