On Wed, Apr 13, 2016 at 2:11 PM, Robert Haas <robertmh...@gmail.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

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