On Tue, Jul 30, 2019 at 4:52 AM Jeevan Ladhe <jeevan.ladhe@enterprisedb.com> wrote: > My only concern was something that we internally treat as invalid, why do > we allow, that as a valid value for that type. While I am not trying to > reinvent the wheel here, I am trying to understand if there had been any > idea behind this and I am missing it.
Well, the word "invalid" can mean more than one thing. Something can be valid or invalid depending on context. I can't have -2 dollars in my wallet, but I could have -2 dollars in my bank account, because the bank will allow me to pay out slightly more money than I actually have on the idea that I will pay them back later (and with interest!). So as an amount of money in my wallet, -2 is invalid, but as an amount of money in my bank account, it is valid.
0/0 is not a valid LSN in the sense that (in current releases) we never write a WAL record there, but it's OK to compute with it. Subtracting '0/0'::pg_lsn seems useful as a way to convert an LSN to an absolute byte-index in the WAL stream.
Thanks Robert for such a nice and detailed explanation.
I now understand why LSN '0/0' can still be useful.
