Re: Logical Replication of sequences

On Fri, 7 Nov 2025 at 14:54, shveta malik <shveta.malik@gmail.com> wrote:
>
> On Fri, Nov 7, 2025 at 10:58 AM vignesh C <vignesh21@gmail.com> wrote:
> >
> >
> > Thanks for pushing the patch, here is a rebased version of the
> > remaining patches.
> >
>
> Please find a few comments on doc patch:
>
> 1)
> +    them. To verify this, compare the
> +    <link linkend="catalog-pg-subscription-rel">pg_subscription_rel</link>.<structfield>srsublsn</structfield>
> +    on the subscriber with the page_lsn obtained from the
> +    <function>pg_get_sequence_data</function> for the sequence on the
> publisher.
>
> Is there a way to give link of 'pg_get_sequence_data' here?

Modified

> 2)
> +   <warning>
> +    <para>
> +     Each sequence caches a block of values (typically 32) in memory before
> +     generating a new WAL record, so its LSN advances only after the entire
> +     cached batch has been consumed. As a result, sequence value
> drift cannot be
> +     detected by comparing LSNs for sequence increments that fall within the
> +     same cached block.
> +    </para>
> +   </warning>
>
> In such a case, shall we mention that compare last_value to see the
> drift? Thoughts?

I was not sure as it might not be very efficient

> 3)
>
> +    To detect this, compare the
> +    <link linkend="catalog-pg-subscription-rel">pg_subscription_rel</link>.<structfield>srsublsn</structfield>
> +    on the subscriber with the page_lsn obtained from the
> +    <function>pg_get_sequence_data</function> for the sequence on the
> publisher.
>
>
> We have mentioned above. But in the example of the same, we do not
> show srsublsn or page_lsn anywhere. Shall we query and show that as
> well?

Updated example

>
> 4)
>          Maximum number of synchronization workers per subscription. This
>          parameter controls the amount of parallelism of the initial data copy
>          during the subscription initialization or when new tables are added.
> +        One additional worker is also needed for sequence synchronization.
>         </para>
>
> Since now the first line is talking only about table-sync, shall we tweak it:
> 'of the initial data copy' --> 'of the initial data copy for tables'

Modified

> 5)
> +        Returns information about the sequence. <literal>last_value</literal>
> +        indicates last sequence value set in sequence by nextval or setval,
>
> last_value can also be set by seq synchronization. Do you think that
> we need to mention that or current info is good enough?

Updated

The attached v20251107_2 version patch has the changes for the same.

Regards,
Vignesh