Re: Odd system-column handling in postgres_fdw join pushdown patch

On Wed, Apr 13, 2016 at 2:11 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> So, clearly that's not good. It should at least be consistent. But
> more than that, the fact that postgres_fdw sets the xmax to 0xffffffff
> is also pretty wacky. We might use such a value as a sentinel for
> some data type, but for transaction IDs that's just some random normal
> transaction ID, and it's NOT coming from t1. I haven't tracked down
> where it *is* coming from yet, but can't imagine it's any place very
> principled.

And, yeah, it's not very principled.

rhaas=# select ft1.xmin, ft1.xmax, ft1.cmin from ft1;
xmin | xmax | cmin
------+------------+-------
96 | 4294967295 | 16392
96 | 4294967295 | 16392
96 | 4294967295 | 16392
96 | 4294967295 | 16392
(4 rows)

What's happening here is that heap_getattr() is being applied to a
HeapTupleHeaderData which contains DatumTupleFields. So 96 is
datum_len_, 4294967295 is the -1 recorded in datum_typmod, and 16392
is the compose type OID recorded in datum_typeid, which happens in
this case to be the OID of ft1. Isn't that special?

It's hard for me to view this as anything other than a bug in
postgres_fdw - which of course means that this open item boils down to
the complaint that the way system columns are handled by join pushdown
isn't bug-compatible with the existing behavior....

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company