pg_subtrans and WAL

My discovery last night of a WAL synchronization error in pg_clog led me
to take a look at pg_subtrans too. I soon realized that in fact we are
not WAL-logging pg_subtrans updates at all: subtransaction start sets up
a pg_subtrans entry but makes no WAL entry for this action.

Seems like this is a problem.

It may be that we do not care because pg_subtrans doesn't have to be
valid after a crash, but I haven't seen any proof of that theory.
And if that theory is correct, then it is a seriously bad design to be
using the same code infrastructure for both pg_clog and pg_subtrans.
Every fsync on pg_subtrans is wasted effort if that is going to be our
approach. We should in fact just delete pg_subtrans and re-init it to
zeroes during postmaster start...

regards, tom lane