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@momjian.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
