Re: concerns around pg_lsn

On Tue, Jul 30, 2019 at 6:06 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Tue, Jul 30, 2019 at 4:52 AM Jeevan Ladhe
> <jeevan(dot)ladhe(at)enterprisedb(dot)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.

Regards,
Jeevan Ladhe