Could you generate the commit-order log by simply registering a commit
hook (RegisterXactCallback(XACT_EVENT_COMMIT)) that writes such a log
somewhere in the data directory? That would work with older versions
too, no server changes required.
It would not get called during recovery, but I believe that would be
sufficient for Slony. You could always batch commits that you don't know
when they committed as if they committed simultaneously.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com