Re: Logical Replication of sequences

On Sun, 22 Jun 2025 at 08:05, vignesh C <vignesh21(at)gmail(dot)com> wrote:
>
> On Thu, 19 Jun 2025 at 11:26, Nisha Moond <nisha(dot)moond412(at)gmail(dot)com> wrote:
> >
> > Hi,
> >
> > Here are my review comments for v20250610 patches:
> >
> > Patch-0005:sequencesync.c
> >
> > 1) report_error_sequences()
> >
> > In case there are both missing and mismatched sequences, the ERROR
> > message logged is -
> >
> > ```
> > 2025-05-28 14:22:19.898 IST [392259] ERROR: logical replication
> > sequence synchronization failed for subscription "subs": sequences
> > ("public"."n84") are missing on the publisher. Additionally,
> > parameters differ for the remote and local sequences ("public.n1")
> > ```
> >
> > I feel this error message is quite long. Would it be possible to split
> > it into ERROR and DETAIL? Also, if feasible, we could consider
> > including a HINT, as was done in previous versions.
> >
> > I explored a few possible ways to log this error with a hint. Attached
> > top-up patch has the suggestion implemented. Please see if it seems
> > okay to consider.
>
> This looks good, merged it.
> > ~~~
> >
> > 2) copy_sequences():
> > + /* Retrieve the sequence object fetched from the publisher */
> > + for (int i = 0; i < batch_size; i++)
> > + {
> > + LogicalRepSequenceInfo *sequence_info =
> > lfirst(list_nth_cell(remotesequences, current_index + i));
> > +
> > + if (!strcmp(sequence_info->nspname, nspname) &&
> > + !strcmp(sequence_info->seqname, seqname))
> > + seqinfo = sequence_info;
> > + }
> >
> > The current logic performs a search through the local sequence list
> > for each sequence fetched from the publisher, repeating the traverse
> > of 100(batch size) length of the list per sequence, which may impact
> > performance.
> >
> > To improve efficiency, we can optimize it by sorting the local list
> > and traverses it only once for matching. Kindly review the
> > implementation in the attached top-up patch and consider merging it if
> > it looks good to you.
>
> Looks good, merged it.
>
> > ~~~
> >
> > 3) copy_sequences():
> > + if (message_level_is_interesting(DEBUG1))
> > + ereport(DEBUG1,
> > + errmsg_internal("logical replication synchronization for
> > subscription \"%s\", sequence \"%s\" has finished",
> > + MySubscription->name,
> > + seqinfo->seqname));
> > +
> > + batch_succeeded_count++;
> > + }
> >
> > The current debug log might be a bit confusing when sequences with the
> > same name exist in different schemas. To improve clarity, we could
> > include the schema name in the message, e.g.,
> > " ... sequence "schema"."sequence" has finished".
>
> Modified
>
> > ~~~~
> >
> > Few minor comments in doc - Patch-0006 : logical-replication.sgml
> >
> > 4)
> > + <para>
> > + To replicate sequences from a publisher to a subscriber, first publish them
> > + using <link linkend="sql-createpublication-params-for-all-sequences">
> > + <command>CREATE PUBLICATION ... FOR ALL SEQUENCES</command></link>.
> > + </para>
> >
> > I think it would be better to use "To synchronize" instead of "To
> > replicate" here, to maintain consistency and avoid confusion between
> > replication and synchronization.
>
> Modified
>
> > 5)
> > + <para>
> > + Update the sequences at the publisher side few times.
> > +<programlisting>
> >
> > /side few /side a few /
> >
> > 6) Can avoid using multiple "or" in the sentences below:
> >
> > 6a)
> > - a change set or replication set. Each publication exists in only
> > one database.
> > + generated from a table or a group of tables or the current state of all
> > + sequences, and might also be described as a change set or replication set
> >
> > / table or a group of tables/ table, a group of tables/
>
> Modified
>
> > 6b)
> > + Publications may currently only contain tables or sequences. Objects must be
> > + added explicitly, except when a publication is created using
> > + <literal>FOR TABLES IN SCHEMA</literal>, or <literal>FOR ALL
> > TABLES</literal>,
> > + or <literal>FOR ALL SEQUENCES</literal>. Unlike tables, the current state of
> >
> > / IN SCHEMA</literal>, or <literal>FOR ALL TABLES/ IN
> > SCHEMA</literal>, <literal>FOR ALL TABLES
>
> Modified
>
> Thanks for the comment, the attached v20250622 version patch has the
> changes for the same.
>
Hi Vignesh,

I tested with all patches applied. I have a comment:

Let consider following case:
On publisher create a publication pub1 on all sequence. publication
has sequence s1. The curr value of s1 is 2
and On subscriber we have subscription on pub1 and sequence s1 has
value 5. Now we run:
"ALTER SUBSCRIPTION sub1 REFRESH PUBLICATION SEQUENCES"

Now on subscriber currval still show '5':
postgres=# select currval('s1');
currval
---------
5
(1 row)

But when we do nextval on s1 on subscriber we get '3'. Which is
correct assuming sequence is synced:
postgres=# select nextval('s1');
nextval
---------
3
(1 row)
postgres=# select currval('s1');
currval
---------
3
(1 row)

Is this behaviour expected? I feel the initial " select
currval('s1');" should have displayed '2'. Thoughts?

Thanks and Regards,
Shlok Kyal