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From: Fabrice Chapuis <[email protected]>
To: [email protected]
Subject: pg_split_walfile_name
Date: Thu, 31 Jul 2025 10:08:25 +0200
Message-ID: <CAA5-nLCE4PyT7Dh5ogNwBc=dwdA2T58Kc7uWHN4fW1GmKPAG4w@mail.gmail.com> (raw)

Hi,

In the documentation, PostgreSQL: Documentation: 17: 9.28. System
Administration Functions
<https://www.postgresql.org/docs/current/functions-admin.html;, 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)

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