Re: ALTER TABLE lock strength reduction patch is unsafe

Simon Riggs <simon@2ndQuadrant.com> writes:
> On Mon, Jan 2, 2012 at 6:06 PM, Noah Misch <noah@leadboat.com> wrote:
>> On Mon, Jan 02, 2012 at 05:09:16PM +0000, Simon Riggs wrote:
>>> Attached patch makes SnapshotNow into an MVCC snapshot,

>> 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?

Almost certainly yes.  For example, a catcache load may invoke catcache
or relcache reload operations on its way to opening the table or index
needed to fetch the desired row.

I think you can only safely do this if each caller has its own snapshot
variable, a la SnapshotDirty, and that's going to be hugely more
invasive.
        regards, tom lane