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

On 2016-06-15 14:50:46 -0400, Robert Haas wrote:
> On Wed, Jun 15, 2016 at 2:45 PM, Alvaro Herrera
> <alvherre@2ndquadrant.com> wrote:
> > Robert Haas wrote:
> >> On Wed, Jun 15, 2016 at 1:44 PM, Kevin Grittner <kgrittn@gmail.com>
> >> wrote:
> >> > The test I showed creates a situation which (to ANALYZE) is
> >> > identical to what you describe -- ANALYZE sees a page with an LSN
> >> > recent enough that it could have been (and actually has been)
> >> > pruned.  Why would it be better for the ANALYZE to fail than to
> >> > complete?
> >>
> >> As I understand it, the reason we need to sometimes give "ERROR:
> >> snapshot too old" after early pruning is because we might otherwise
> >> give the wrong answer.
> >
> > So what constitutes "the wrong answer"?  A regular transaction reading a
> > page and not finding a tuple that should have been there but was
> > removed, is a serious problem and should be aborted.  For ANALYZE it may
> > not matter a great deal.  Some very old tuple that might have been
> > chosen for the sample is not there; a different tuple is chosen instead,
> > so the stats might be slightly difference.  No big deal.
> >
> > Maybe it is possible to get into trouble if you're taking a sample for
> > an expression index.
>
> The expression index case is the one to worry about; if there is a
> problem, that's where it is.  What bothers me is that a function used
> in an expression index could do anything at all - it can read any
> table in the database.

Isn't that also a problem around fetching toast tuples? As we don't
TestForOldSnapshot_impl() for toast, We might fetch a toast tuple which
since have been re-purposed for a datum of a different type. Which can
have arbitrarily bad consequences afaics.

Andres