public inbox for [email protected]  

help / color / mirror / Atom feed

From: Fabrice Chapuis <[email protected]>
To: Bruce Momjian <[email protected]>
Cc: [email protected]
Subject: Re: pg_split_walfile_name
Date: Wed, 13 Aug 2025 12:06:15 +0200
Message-ID: <CAA5-nLAhDhC=HTrqckygMv0CagVs39j0jiGgrGRUPDd=PFB4OA@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <CAA5-nLCE4PyT7Dh5ogNwBc=dwdA2T58Kc7uWHN4fW1GmKPAG4w@mail.gmail.com>
	<[email protected]>

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 <[email protected]> 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  <[email protected]>        https://momjian.us
>   EDB                                      https://enterprisedb.com
>
>   Do not let urgent matters crowd out time for investment in the future.
>

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected], [email protected]
  Subject: Re: pg_split_walfile_name
  In-Reply-To: <CAA5-nLAhDhC=HTrqckygMv0CagVs39j0jiGgrGRUPDd=PFB4OA@mail.gmail.com>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox