Re: pg_walfile_name uses XLByteToPrevSeg

At Fri, 4 Feb 2022 14:50:57 -0800, Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote in
> On Fri, Feb 04, 2022 at 09:17:54AM -0500, Robert Haas wrote:
> > On Fri, Feb 4, 2022 at 9:05 AM Ashutosh Bapat
> > <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> wrote:
> >> And it gives some surprising results as well
> >> ---
> >> #select pg_walfile_name('0/0'::pg_lsn);
> >> pg_walfile_name
> >> --------------------------
> >> 00000001FFFFFFFF000000FF
> >> (1 row)
> >> ----
> >
> > Yeah, that seems wrong.
>
> It looks like it's been this way for a while (704ddaa).
> pg_walfile_name_offset() has the following comment:
>
> * Note that a location exactly at a segment boundary is taken to be in
> * the previous segment. This is usually the right thing, since the
> * expected usage is to determine which xlog file(s) are ready to archive.
>
> I see a couple of discussions about this as well [0] [1].
>
> [0] https://www.postgresql.org/message-id/flat/1154384790.3226.21.camel%40localhost.localdomain
> [1] https://www.postgresql.org/message-id/flat/15952.1154827205%40sss.pgh.pa.us

Yes, its the deliberate choice of design, or a kind of
questionable-but-unoverturnable decision. I think there are many
external tools conscious of this behavior.

It is also described in the documentation.

https://www.postgresql.org/docs/current/functions-admin.html
> When the given write-ahead log location is exactly at a write-ahead
> log file boundary, both these functions return the name of the
> preceding write-ahead log file. This is usually the desired behavior
> for managing write-ahead log archiving behavior, since the preceding
> file is the last one that currently needs to be archived.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center