Re: Replication sequence - Mailing list pgsql-general

From Jehan-Guillaume de Rorthais
Subject Re: Replication sequence
Date
Msg-id 20210216105145.03b5943b@firost
Whole thread Raw
In response to Replication sequence  (Paolo Saudin <paolosaudin@gmail.com>)
Responses Re: Replication sequence  (Paolo Saudin <paolosaudin@gmail.com>)
List pgsql-general
On Mon, 15 Feb 2021 18:55:14 +0100
Paolo Saudin <paolosaudin@gmail.com> wrote:

> Hi all,
> I have two servers, a primary and a secondary one with a streaming replica
> setup.
> Today I noticed that some sequences are not lined-up, the replica ones are
> well ahead, while the records number is the same. How is it possible?

This is because sequences are not WAL logged on every nextval() call, but by
batch of sequence cache+32 values. As standbys are fed with WAL stream from
primary, their sequences appears jump, then stall, then jump, etc.

See:

https://git.postgresql.org/cgit/postgresql.git/tree/src/backend/commands/sequence.c?id=f672df5fdd22dac14c98d0a0bf5bbaa6ab17f8a5#n52


https://git.postgresql.org/cgit/postgresql.git/tree/src/backend/commands/sequence.c?id=f672df5fdd22dac14c98d0a0bf5bbaa6ab17f8a5#n661

I'm not sure if this is documented somewhere... ?



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