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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