Re: Re: [COMMITTERS] pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On 2016-07-13 10:06:52 -0500, Kevin Grittner wrote:
> On Wed, Jul 13, 2016 at 7:57 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > On Tue, Jul 12, 2016 at 8:34 PM, Kevin Grittner <kgrittn(at)gmail(dot)com> wrote:
> >> On Fri, Jul 8, 2016 at 1:52 PM, Andres Freund <andres(at)anarazel(dot)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?