Cannot COPY xmin?

Hi. I just tried adding the xmin pseudo-column to my COPY,
and I get an error. Using a regular SELECT works fine.

Is that normal? that's a `COPY tab(xmin, cols...) ...`

Do I need to switch to a `COPY (select xmin, cols... from tab) ...` to
access xmin?

And if I do that, any performance implications for the COPY?
The reason I'm using COPY is performance and its "streaming" behavior.
(modulo the "big row" non-streaming issue from my as-yet unanswered post
from today).
I'm thus interested in how a `COPY tab(cols...)` differs from a `COPY
(select cols from tab ...)` in terms of implementation and performance.

Thanks, --DD