"Jim C. Nasby" <jnasby(at)pervasive(dot)com> writes:
> I can't seem to find info about this in the docs, or on the web.
http://developer.postgresql.org/docs/postgres/storage-page-layout.html
Currently, when a tuple is reclaimed by VACUUM, we just mark its item
pointer as unused (and hence recyclable). I think it might be safe to
decrease pd_lower if there are unused pointers at the end of the page's
pointer array, but we don't currently do that. In any case there could
still be unused pointers embedded within the array.
regards, tom lane