From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@enterprisedb.com>
cc: Michael Paquier <michael.paquier@gmail.com>,
        Andrew Gierth <rhodiumtoad@postgresql.org>,
        PostgreSQL mailing lists <pgsql-bugs@postgresql.org>,
        Kevin Grittner <kgrittn@gmail.com>
Subject: Re: BUG #14808: V10-beta4, backend abort
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Comments: In-reply-to Thomas Munro <thomas.munro@enterprisedb.com>
	message dated "Thu, 14 Sep 2017 11:00:40 +1200"
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Date: Wed, 13 Sep 2017 19:45:46 -0400
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Thomas Munro <thomas.munro@enterprisedb.com> writes:
> On Thu, Sep 14, 2017 at 10:44 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> BTW, as I'm digging around in trigger.c, I can't help noticing that
>> it provides a single "fdw_tuplestore" per trigger query level (a/k/a
>> trigger execution context).  I've not tried to test this, but it
>> sure looks like a wCTE like your example above, directed at two
>> separate foreign tables with triggers, would fail for exactly the
>> same reason.  That'd be a bug of pretty long standing.

> I had the impression that that fdw_tuplestore was doing something a
> bit sneaky that actually works out OK: tuples get enqueued and later
> dequeued in exactly the same sequence as the after row trigger events
> that need them, so even though it seems to violate at least the POLA
> if not the spirit of tuplestores by storing tuples of potentially
> different types in one tuplestore, nothing bad should happen.

Oh?  Now my fear level is up to 11, because it is completely trivial to
cause triggers to fire in a different order than they were enqueued.
All you need is a mix of deferrable and nondeferrable triggers.

In fact, it also seems entirely broken that a per-query-level tuplestore
is being used at all, because deferrable triggers might not get fired
until some outer query level.

[ Pokes around... ]  Hm, looks like we get around that by forbidding
constraint triggers on foreign tables, but I don't see anything in the
CREATE TRIGGER man page saying that there's such a prohibition.  And
there's certainly no comments in the source code explaining this rickety
set of requirements :-(

			regards, tom lane


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