On Wed, Oct 4, 2017 at 11:00 AM, Wood, Dan <hexp...@amazon.com> wrote:
> The early “break;” here is likely the xmin frozen reason as I found in the 
> other loop.

It looks that way.

Since we're already very defensive when it comes to this xmin/xmax
matching business, and we're defensive while following an update chain
more generally, I wonder if we should be checking
HeapTupleHeaderIsSpeculative() on versions >= 9.5 (versions with ON
CONFLICT DO UPDATE, where t_ctid block number might actually be a
speculative insertion token). Or, at least acknowledging that case in
comments. I remember expressing concern that something like this was
possible at the time that that went in.

We certainly don't want to have heap_abort_speculative() "super
delete" the wrong tuple in the event of item pointer recycling. There
are at least defensive sanity checks that would turn that into an
error within heap_abort_speculative(), so it wouldn't be a disaster if
it was attempted. I don't think that it's possible in practice, and
maybe it's sufficient that routines like heap_get_latest_tid() check
for a sane item offset, which will discriminate against
SpecTokenOffsetNumber, which must be well out of range for ItemIds on
the page. Could be worth a comment.

(I guess that heap_prune_chain() wouldn't need to be changed if we
decide to add such comments, because the speculative tuple ItemId is
going to be skipped over due to being ItemIdIsUsed() before we even
get there.)
Peter Geoghegan

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