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From: Fabrice Chapuis <fabrice636861@gmail.com>
Date: Thu, 31 Jul 2025 10:08:25 +0200
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 <CAA5-nLCE4PyT7Dh5ogNwBc=dwdA2T58Kc7uWHN4fW1GmKPAG4w@mail.gmail.com>
Subject: pg_split_walfile_name
To: pgsql-admin@lists.postgresql.org
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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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<div dir=3D"ltr"><div dir=3D"ltr"><div dir=3D"ltr"><p style=3D"box-sizing:b=
order-box;margin-top:0px;margin-bottom:1rem;color:rgb(0,0,0);font-family:&q=
uot;Open Sans&quot;,sans-serif;font-size:14.4px"><code class=3D"gmail-funct=
ion" style=3D"box-sizing:border-box;font-family:monospace,monospace;font-si=
ze:1em;background-color:rgb(248,249,250);border-radius:0.25rem;margin:0.6re=
m 0px">Hi,</code></p><p style=3D"box-sizing:border-box;margin-top:0px;margi=
n-bottom:1rem;color:rgb(0,0,0);font-family:&quot;Open Sans&quot;,sans-serif=
;font-size:14.4px"><code class=3D"gmail-function" style=3D"box-sizing:borde=
r-box;font-family:monospace,monospace;font-size:1em;background-color:rgb(24=
8,249,250);border-radius:0.25rem;margin:0.6rem 0px">In the documentation,=
=C2=A0</code><a href=3D"https://www.postgresql.org/docs/current/functions-a=
dmin.html" style=3D"font-family:Arial,Helvetica,sans-serif;font-size:small"=
>PostgreSQL: Documentation: 17: 9.28.=C2=A0System Administration Functions<=
/a>,=C2=A0<span style=3D"background-color:rgb(248,249,250);font-family:mono=
space,monospace;font-size:1em">I do not understand the position 100C in the=
 wal filename and how the result could be C001?</span></p><p style=3D"box-s=
izing:border-box;margin-top:0px;margin-bottom:1rem;color:rgb(0,0,0);font-fa=
mily:&quot;Open Sans&quot;,sans-serif;font-size:14.4px"><span style=3D"back=
ground-color:rgb(248,249,250);font-family:monospace,monospace;font-size:1em=
">Any idea?</span></p><p style=3D"box-sizing:border-box;margin-top:0px;marg=
in-bottom:1rem;color:rgb(0,0,0);font-family:&quot;Open Sans&quot;,sans-seri=
f;font-size:14.4px">Regards,</p><p style=3D"box-sizing:border-box;margin-to=
p:0px;margin-bottom:1rem;color:rgb(0,0,0);font-family:&quot;Open Sans&quot;=
,sans-serif;font-size:14.4px">Fabrice</p><p style=3D"box-sizing:border-box;=
margin-top:0px;margin-bottom:1rem;color:rgb(0,0,0);font-family:&quot;Open S=
ans&quot;,sans-serif;font-size:14.4px"><code class=3D"gmail-function" style=
=3D"box-sizing:border-box;font-family:monospace,monospace;font-size:1em;bac=
kground-color:rgb(248,249,250);border-radius:0.25rem;margin:0.6rem 0px">pg_=
split_walfile_name</code>=C2=A0is useful to compute a=C2=A0<acronym class=
=3D"gmail-acronym" style=3D"box-sizing:border-box">LSN</acronym>=C2=A0from =
a file offset and WAL file name, for example:</p><pre class=3D"gmail-progra=
mlisting" style=3D"box-sizing:border-box;font-family:monospace,monospace;fo=
nt-size:14.4px;margin:1rem 0px;overflow:auto;color:rgb(0,0,0);border-radius=
:0.25rem;padding:0.8rem;border-color:rgb(206,212,218);border-style:solid;bo=
rder-width:1px"><span style=3D"background-color:rgb(248,249,250)">postgres=
=3D# \set file_name &#39;000000010000000</span><span style=3D"background-co=
lor:rgb(255,0,0)">100C</span><span style=3D"background-color:rgb(248,249,25=
0)">000AB&#39;
postgres=3D# \set offset 256
postgres=3D# SELECT &#39;0/0&#39;::pg_lsn + pd.segment_number * ps.setting:=
:int + :offset AS lsn
  FROM pg_split_walfile_name(:&#39;file_name&#39;) pd,
       pg_show_all_settings() ps
  WHERE <a href=3D"http://ps.name">ps.name</a> =3D &#39;wal_segment_size=
9;;
      lsn
---------------
 </span><span style=3D"background-color:rgb(255,0,0)">C001</span><span styl=
e=3D"background-color:rgb(248,249,250)">/AB000100
(1 row)</span></pre></div></div></div>

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