Re: Logical Replication of sequences

Hi,

On Tue, Jun 4, 2024 at 5:40 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> Even if we decode it periodically (say each time we decode the
> checkpoint record) then also we need to send the entire set of
> sequences at shutdown. This is because the sequences may have changed
> from the last time we sent them.

Agree. How about decoding and sending only the sequences that are
changed from the last time when they were sent? I know it requires a
bit of tracking and more work, but all I'm looking for is to reduce
the amount of work that walsenders need to do during the shutdown.

Having said that, I like the idea of letting the user sync the
sequences via ALTER SUBSCRIPTION command and not weave the logic into
the shutdown checkpoint path. As Peter Eisentraut said here
https://www.postgresql.org/message-id/42e5cb35-4aeb-4f58-8091-90619c7c3ecc%40eisentraut.org,
this can be a good starting point to get going.

> > I can imagine a case where there are tens
> > of thousands of sequences in a production server, and surely decoding
> > and sending them just during the shutdown can take a lot of time
> > hampering the overall server uptime.
>
> It is possible but we will send only the sequences that belong to
> publications for which walsender is supposed to send the required
> data.

Right, but what if all the publication tables can have tens of
thousands of sequences.

> Now, we can also imagine providing option 2 (Alter Subscription
> ... Replicate Sequences) so that users can replicate sequences before
> shutdown and then disable the subscriptions so that there won't be a
> corresponding walsender.

As stated above, I like this idea to start with.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com