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help / color / mirror / Atom feedFrom: Bruce Momjian <[email protected]>
To: Fabrice Chapuis <[email protected]>
Cc: [email protected]
Subject: Re: pg_split_walfile_name
Date: Tue, 12 Aug 2025 13:34:56 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAA5-nLCE4PyT7Dh5ogNwBc=dwdA2T58Kc7uWHN4fW1GmKPAG4w@mail.gmail.com>
References: <CAA5-nLCE4PyT7Dh5ogNwBc=dwdA2T58Kc7uWHN4fW1GmKPAG4w@mail.gmail.com>
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
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