FDW system columns

I notice that there's some weird info coming out of the system columns
on any FDW:

test=# select tableoid, ctid, xmin, xmax, cmin, cmax, * from dict limit 12;tableoid |      ctid      | xmin |    xmax
| cmin  | cmax  |   words
 
----------+----------------+------+------------+-------+-------+-----------   16428 | (4294967295,0) |  104 |
4294967295| 16430 | 16430 | A   16428 | (4294967295,0) |  104 | 4294967295 | 16430 | 16430 | a   16428 | (4294967295,0)
| 108 | 4294967295 | 16430 | 16430 | aa   16428 | (4294967295,0) |  112 | 4294967295 | 16430 | 16430 | aal   16428 |
(4294967295,0)|  120 | 4294967295 | 16430 | 16430 | aalii   16428 | (4294967295,0) |  112 | 4294967295 | 16430 | 16430
|aam   16428 | (4294967295,0) |  116 | 4294967295 | 16430 | 16430 | Aani   16428 | (4294967295,0) |  132 | 4294967295 |
16430| 16430 | aardvark   16428 | (4294967295,0) |  132 | 4294967295 | 16430 | 16430 | aardwolf   16428 |
(4294967295,0)|  120 | 4294967295 | 16430 | 16430 | Aaron   16428 | (4294967295,0) |  128 | 4294967295 | 16430 | 16430
|Aaronic   16428 | (4294967295,0) |  136 | 4294967295 | 16430 | 16430 | Aaronical
 
(12 rows)

That's file_fdw.  On the not-yet-ready pgsql_fdw:

test=# select tableoid, ctid, xmin, xmax, cmin, cmax from cows limit 5;tableoid |      ctid      | xmin | xmax | cmin |
cmax
----------+----------------+------+------+------+------   16406 | (4294967295,0) |    0 |    0 |    0 |    0   16406 |
(4294967295,0)|    0 |    0 |    0 |    0   16406 | (4294967295,0) |    0 |    0 |    0 |    0   16406 | (4294967295,0)
|   0 |    0 |    0 |    0   16406 | (4294967295,0) |    0 |    0 |    0 |    0
 
(5 rows)

So the ctid is always 2^32-1.  Bit weird, but probably explainable.
But xmin on the file_fdw result is odd.  Why are these all over the
place?

-- 
Thom Brown
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