On 2016/04/14 4:57, Robert Haas wrote:
On Wed, Apr 13, 2016 at 2:36 PM, Robert Haas <robertmh...@gmail.com> wrote:
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....
OK, here's a patch. What I did is:
Thank you for taking care of this.
1. For a regular FDW scan, zero the xmin, xmax, and cid of the tuple
before returning it from postgres_fdw, so that we don't expose the
datum-tuple fields. I can't see any reason this isn't safe, but I
might be missing something.
I'm not sure that is really safe.
2. When a join is pushed down, deparse system columns using something
like "CASE WHEN r1.* IS NOT NULL THEN 0 END", except for the table OID
column, which gets deparsed with the table OID in place of 0. This
delivers the correct behavior in the presence of outer joins.
I think that that would cause useless data transfer for such culumns.
Why not set values locally for such columns?
Best regards,
Etsuro Fujita
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