Re: [sqlsmith] Crash in GetOldestSnapshot()

>>>>> "Amit" == Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> writes:

Amit> Sure, that is the reason of crash, but even if we do that it will
Amit> lead to an error "no known snapshots". Here, what is going on is
Amit> that we initialized toast snapshot when there is no active
Amit> snapshot in the backend, so GetOldestSnapshot() won't return any
Amit> snapshot.

Hmm.

So this happens because RETURNING queries run to completion immediately
and populate a tuplestore with the results, and the portal then fetches
from the tuplestore to send to the destination. The assumption is that
the tuplestore output can be processed without needing a snapshot, which
obviously is not true now if it contains toasted data.

In a similar case in the past involving holdable cursors, the solution
was to detoast _before_ storing in the tuplestore (see
PersistHoldablePortal). I guess the question now is, under what
circumstances is it now allowable to detoast a datum with no active
snapshot? (Wouldn't it be necessary in such a case to know what the
oldest snapshot ever used in the transaction was?)

Amit> I think for such situations, we need to initialize the lsn and
Amit> whenTaken of ToastSnapshot as we do in GetSnapshotData() [1].

Would that not give a too-recent LSN, resulting in possibly failing to
fetch the toast rows?

--
Andrew (irc:RhodiumToad)