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

On 2016-06-16 13:44:34 -0400, Robert Haas wrote:
> On Thu, Jun 16, 2016 at 12:54 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >> The root of my confusion is: if we prune a tuple, we'll bump the page
> >> LSN, so any session that is still referencing that tuple will error
> >> out as soon as it touches the page on which that tuple used to exist.
> >
> > Right. On the main table. But we don't peform that check on the toast
> > table/pages. So if we prune toast tuples, which are still referenced by
> > (unvacuumed) main relation, we can get into trouble.
>
> OK, if it's true that we don't perform that check on the TOAST table,
> then I agree there's a potential problem there. I don't immediately
> know where in the code to look to check that.

static inline void
TestForOldSnapshot(Snapshot snapshot, Relation relation, Page page)
{
Assert(relation != NULL);

if (old_snapshot_threshold >= 0
&& (snapshot) != NULL
&& (snapshot)->satisfies == HeapTupleSatisfiesMVCC
&& !XLogRecPtrIsInvalid((snapshot)->lsn)
&& PageGetLSN(page) > (snapshot)->lsn)
TestForOldSnapshot_impl(snapshot, relation);
}

The relevant part is the HeapTupleSatisfiesMVCC check, we're using
SatisfiesToast for toast lookups.

FWIW, I just tried to reproduce this with old_snapshot_threshold = 0 -
but ran into the problem that I couldn't get it to vacuum anything away
(neither main nor toast rel). That appears to be
if (old_snapshot_threshold == 0)
{
if (TransactionIdPrecedes(latest_xmin, MyPgXact->xmin)
&& TransactionIdFollows(latest_xmin, xlimit))
xlimit = latest_xmin;
because latest_xmin always is equal to MyPgXact->xmin, which is actually
kinda unsurprising?

Regards,

Andres