Date: Wed, 25 Mar 2026 16:15:20 +0000 (GMT)
From: PALAYRET Jacques <jacques.palayret@meteo.fr>
To: pgsql-general@lists.postgresql.org
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Subject: Re: Logical replication in PostgreSQL Amount of subscriber vs
 publisher WALs Vacuuming
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Thread-Topic: Logical replication in PostgreSQL Amount of subscriber vs publisher WALs Vacuuming
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Hello,=20

Vacuuming seems to explain these differences.=20
Especially for large tables, vacuuming can generate many WAL.=20

Regards=20

De: "PALAYRET Jacques" <jacques.palayret@meteo.fr>=20
=C3=80: pgsql-general@lists.postgresql.org=20
Envoy=C3=A9: Mardi 24 Mars 2026 15:06:23=20
Objet: Logical replication in PostgreSQL Amount of subscriber vs publisher =
WALs=20

Hello,=20

The amount of WAL generated by an SQL INSERT or UPDATE statement applied to=
 a table with multiple indexes can be much greater than the size of the tab=
le (table + index).=20
For example, an INSERT statement in an empty table (with 3 indexes) can gen=
erate WALs twice the size of the table (table + index).=20
This difference (even for an INSERT) may seem surprising, but it's understa=
ndable.=20

What's less intuitive is that, according to my tests, with logical replicat=
ion in PostgreSQL, the amount of WAL generated by an SQL statement can be v=
ery different between the subscriber server (the replica) and the publisher=
 server (the provider).=20
Is this accurate? Sometimes 1.5 or 2 times greater?=20

Regards=20
----- M=C3=A9t=C3=A9o-France -----=20
PALAYRET Jacques=20
DCSC/GDC=20
jacques.palayret@meteo.fr=20
Fixe : +33 561078319=20


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<html><body><div id=3D"zimbraEditorContainer" style=3D"font-family: garamon=
d, new york, times, serif; font-size: 12pt; color: #000000" class=3D"4"><di=
v>Hello,</div><div><br data-mce-bogus=3D"1"></div><div>Vacuuming seems to e=
xplain these differences.<br>Especially for large tables, vacuuming can gen=
erate many WAL.</div><div><br data-mce-bogus=3D"1"></div><div>Regards<br></=
div><hr id=3D"zwchr" data-marker=3D"__DIVIDER__"><div data-marker=3D"__HEAD=
ERS__"><b>De: </b>"PALAYRET Jacques" &lt;jacques.palayret@meteo.fr&gt;<br><=
b>=C3=80: </b>pgsql-general@lists.postgresql.org<br><b>Envoy=C3=A9: </b>Mar=
di 24 Mars 2026 15:06:23<br><b>Objet: </b>Logical replication in PostgreSQL=
 Amount of subscriber vs publisher WALs<br></div><div><br></div><div data-m=
arker=3D"__QUOTED_TEXT__"><div style=3D"font-family:'garamond' , 'new york'=
 , 'times' , serif;font-size:12pt;color:#000000"><div>Hello,</div><br><div>=
The amount of WAL generated by an SQL INSERT or UPDATE statement applied to=
 a table with multiple indexes can be much greater than the size of the tab=
le (table + index).<br>For example, an INSERT statement in an empty table (=
with 3 indexes) can generate WALs twice the size of the table (table + inde=
x).<br>This difference (even for an INSERT) may seem surprising, but it's u=
nderstandable.</div><div><br>What's less intuitive is that, according to my=
 tests, with logical replication in PostgreSQL, the amount of WAL generated=
 by an SQL statement can be very different between the subscriber server (t=
he replica) and the publisher server (the provider).<br>Is this accurate? S=
ometimes 1.5 or 2 times greater?</div><br><div>Regards<br></div><div>----- =
M=C3=A9t=C3=A9o-France -----<br>PALAYRET Jacques<br>DCSC/GDC<br>jacques.pal=
ayret@meteo.fr<br>Fixe&nbsp;:&nbsp;+33 561078319</div></div><br></div></div=
></body></html>
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