Re: pg_walfile_name uses XLByteToPrevSeg

At Mon, 07 Feb 2022 13:21:53 +0900 (JST), Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> wrote in
> 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.

I forgot to mentino, but I don't think we need to handle the
wrap-around case of the function.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center