On Wed, Jul 17, 2019 at 3:44 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> Right, actually I got that point. But, I was thinking that we are
> wasting one logno from undo log addressing space no?. Instead, if we
> can keep it attached to the slot and somehow manage to add to the free
> list then the same logno can be used by someone else?
We can never reuse log numbers. UndoRecPtr values containing that log
number could exist in permanent storage anywhere (zheap, zedstore etc)
and must appear to be discarded forever if anyone asks. Now, it so
happens that the current coding in zheap has fxid + urp for each
transaction slot and always checks the fxid first so it probably
wouldn't ask about discarded urps too much, but I don't think that's
policy is a requirement and the undo layer can't count on it. I think
I heard that zedstore is planning to check urp only.
--
Thomas Munro
https://enterprisedb.com