Re: pg_walfile_name uses XLByteToPrevSeg

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

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Nathan Bossart
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