On Thu, Sep 14, 2017 at 11:00 AM, Thomas Munro
<thomas.munro@enterprisedb.com> wrote:
> 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. I
> suppose it was by copying that coding that Kevin finished up with the
> initial bug that wCTEs mix stuff from different wCTEs and it all blows
> up, because it has no similar sequencing trick: triggers with
> transition tables were seeing all of them, and they weren't even
> guaranteed to be of the right type.
Incidentally, understanding that made me wonder why we don't have a
binary chunk-oriented in-memory-up-to-some-size-then-spill-to-disk
spooling mechanism that could be used for the trigger queue itself
(which currently doesn't know how to spill to disk and therefore can
take your server out), including holding these tuple images directly
(instead of spilling just the tuples in synchronised order with the
in-memory trigger queue).
--
Thomas Munro
http://www.enterprisedb.com
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