"Michael P. Soulier" <michael_soulier(at)mitel(dot)com> writes:
> But I don't understand why this was required. What's wrong with adding a
> column and copying data into it in a transaction?
Nothing. The problem apparently is that you've got deferred AFTER
triggers on that table, so the UPDATE commands have left unprocessed
trigger events behind, and the system can't be sure that those events
would still be sensible to fire after doing further ALTERs on the table.
*Why* you've got such triggers is not apparent from what you've told us.
regards, tom lane