Re: ALTER TABLE lock strength reduction patch is unsafe

Excerpts from Noah Misch's message of lun ene 02 16:25:25 -0300 2012:
> On Mon, Jan 02, 2012 at 06:41:31PM +0000, Simon Riggs wrote:
> > 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, 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?)
>
> I hacked up your patch a bit, as attached, to emit a WARNING for any nested
> use of SnapshotNow.  This made 97/127 test files fail.  As one example,
> RelationBuildRuleLock() does TextDatumGetCString() for every tuple of its
> SnapshotNow scan.  That may need a detoast, which itself runs a scan.

Uh, I thought detoasting had its own visibility test function .. I mean,
otherwise, what is HeapTupleSatisfiesToast for?

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Álvaro Herrera <alvherre@commandprompt.com>
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