From: Andrew Gierth <andrew@tao11.riddles.org.uk>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Andreas Seltenreich <seltenreich@gmx.de>,
  pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: [sqlsmith] Crash in GetOldestSnapshot()
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 <CAA4eK1LiRxT_iPfJ=uK13tLCtaGAQqn_YOY=6zCBqoi2mW333A@mail.gmail.com>
	(Amit Kapila's message of "Sat, 6 Aug 2016 18:03:47 +0530")
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	<CAA4eK1LiRxT_iPfJ=uK13tLCtaGAQqn_YOY=6zCBqoi2mW333A@mail.gmail.com>
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Date: Sat, 06 Aug 2016 14:00:51 +0100
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>>>>> "Amit" == Amit Kapila <amit.kapila16@gmail.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)


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