Re: pg_split_walfile_name

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


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