Re: pg_split_walfile_name

select * FROM pg_split_walfile_name(:'file_name') pd;
+-[ RECORD 1 ]---+----------+
| segment_number | 12583339 |
| timeline_id | 1 |
+----------------+----------+

Yes, number of sgements 12583339 correspond to C001AB in hexdecimal

thanks for answering

Fabrice

On Tue, Aug 12, 2025 at 7:34 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:

> On Thu, Jul 31, 2025 at 10:08:25AM +0200, Fabrice Chapuis wrote:
> > Hi,
> >
> > In the documentation, PostgreSQL: Documentation: 17: 9.28. System
> > Administration Functions, I do not understand the position 100C in the
> wal
> > filename and how the result could be C001?
> >
> > Any idea?
> >
> > Regards,
> >
> > Fabrice
> >
> > pg_split_walfile_name is useful to compute a LSN from a file offset and
> WAL
> > file name, for example:
> >
> > postgres=# \set file_name '000000010000000100C000AB'
> > postgres=# \set offset 256
> > postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int +
> :offset AS lsn
> > FROM pg_split_walfile_name(:'file_name') pd,
> > pg_show_all_settings() ps
> > WHERE ps.name = 'wal_segment_size';
> > lsn
> > ---------------
> > C001/AB000100
> > (1 row)
>
> I think your problem is that you are converting the WAL segment/offset
> to a number of bytes by multiplying by wal_segment_size, and then you
> are casting it to an LSN, which assumes time line/segment number; try
> this:
>
> SELECT to_hex((pd.segment_number * ps.setting::int +
> :offset)::bigint) AS lsn
> FROM pg_split_walfile_name(:'file_name') pd,
> pg_show_all_settings() ps
> WHERE ps.name = 'wal_segment_size';
> lsn
> --------------
> c001ab000100
>
>
> --
> Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
> EDB https://enterprisedb.com
>
> Do not let urgent matters crowd out time for investment in the future.
>