On Wed, Jul 13, 2016 at 03:57:02PM -0500, Kevin Grittner wrote:
> On Wed, Jul 13, 2016 at 12:48 PM, Andres Freund <and...@anarazel.de> wrote:
> > On 2016-07-13 10:06:52 -0500, Kevin Grittner wrote:
> >> On Wed, Jul 13, 2016 at 7:57 AM, Amit Kapila <amit.kapil...@gmail.com> 
> >> wrote:
> >>> On Tue, Jul 12, 2016 at 8:34 PM, Kevin Grittner <kgri...@gmail.com> wrote:
> >>>> On Fri, Jul 8, 2016 at 1:52 PM, Andres Freund <and...@anarazel.de> wrote:
> >>>>
> >>>>> I'm a bit confused, why aren't we simply adding LSN interlock
> >>>>> checks for toast? Doesn't look that hard? Seems like a much more
> >>>>> natural course of fixing this issue?
> >>>>
> >>>> I took some time trying to see what you have in mind, and I'm
> >>>> really not "getting it".
> >>>
> >>> Isn't it possible if we initialize lsn and whenTaken in SnapshotToast
> >>> when old_snapshot_threshold > 0 and add a check for
> >>> HeapTupleSatisfiesToast in TestForOldSnapshot()?
> >>
> >> With that approach, how will we know *not* to generate an error
> >> when reading the chain of tuples for a value we are deleting.  Or
> >> positioning to modify an index on toast data.  Etc., etc. etc.
> >
> > I'm not following. How is that different in the toast case than in the
> > heap case?
> 
> A short answer is that a normal table's heap doesn't go through
> systable_getnext_ordered().  That function is used both for cases
> where the check should not be made, like toast_delete_datum(), and
> cases where it should, like toast_fetch_datum().
> 
> Since this keeps coming up, I'll produce a patch this way.  I'm
> skeptical, but maybe it will look better than I think it will.  I
> should be able to post that by Friday.

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