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Logical replication in PostgreSQL Amount of subscriber vs publisher WALs
- Jump to comment-1PALAYRET Jacques<jacques.palayret@meteo.fr>Mar 24, 2026, 2:06 PM UTCHello,
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 table (table + index).
For example, an INSERT statement in an empty table (with 3 indexes) can generate WALs twice the size of the table (table + index).
This difference (even for an INSERT) may seem surprising, but it's understandable.
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 (the replica) and the publisher server (the provider).
Is this accurate? Sometimes 1.5 or 2 times greater?
Regards
----- Météo-France -----
PALAYRET Jacques
DCSC/GDC
jacques.palayret@meteo.fr
Fixe : +33 561078319- Jump to comment-1PALAYRET Jacques<jacques.palayret@meteo.fr>Mar 25, 2026, 4:15 PM UTCHello,
Vacuuming seems to explain these differences.
Especially for large tables, vacuuming can generate many WAL.
Regards
De: "PALAYRET Jacques" <jacques.palayret@meteo.fr>
À: pgsql-general@lists.postgresql.org
Envoyé: Mardi 24 Mars 2026 15:06:23
Objet: Logical replication in PostgreSQL Amount of subscriber vs publisher WALs
Hello,
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 table (table + index).
For example, an INSERT statement in an empty table (with 3 indexes) can generate WALs twice the size of the table (table + index).
This difference (even for an INSERT) may seem surprising, but it's understandable.
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 (the replica) and the publisher server (the provider).
Is this accurate? Sometimes 1.5 or 2 times greater?
Regards
----- Météo-France -----
PALAYRET Jacques
DCSC/GDC
jacques.palayret@meteo.fr
Fixe : +33 561078319