Florian Weimer <fweimer(at)bfk(dot)de> writes:
> In theory, this should be possible (especially if you haven't switched
> off full page writes).
Not really --- the WAL records are not designed to carry full
information about the preceding state of the page, so you can't use them
to undo. (Example: a DELETE record says which tuple was deleted, but
not what was in it.)
regards, tom lane