Re: Logical Replication of sequences

On Wed, Jul 24, 2024 at 11:52 AM shveta malik <shveta.malik@gmail.com> wrote:
>
> On Wed, Jul 24, 2024 at 9:17 AM Peter Smith <smithpb2250@gmail.com> wrote:
> >
>
> I had a look at patches v20240720* (considering these as the latest
> one) and tried to do some basic testing (WIP). Few comments:
>
> 1)
> I see 'last_value' is updated wrongly after create-sub.  Steps:
>
> -----------
> pub:
> CREATE SEQUENCE myseq0 INCREMENT 5 START 100;
> SELECT nextval('myseq0');
> SELECT nextval('myseq0');
> --last_value on pub is 105
> select * from pg_sequences;
> create publication pub1 for all tables, sequences;
>
> Sub:
> CREATE SEQUENCE myseq0 INCREMENT 5 START 100;
> create subscription sub1 connection 'dbname=postgres host=localhost
> user=shveta port=5433' publication pub1;
>
> --check 'r' state is reached
> select pc.relname, pr.srsubstate, pr.srsublsn from pg_subscription_rel
> pr, pg_class pc where (pr.srrelid = pc.oid);
>
> --check 'last_value', it shows some random value as 136
> select * from pg_sequences;

Okay, I see that in fetch_remote_sequence_data(), we are inserting
'last_value + log_cnt' fetched from remote as 'last_val' on subscriber
and thus leading to above behaviour. I did not understand why this is
done? This may result into issue when we insert data into a table with
identity column on subscriber (whose internal sequence is replicated);
the identity column in this case will end up having wrong value.

thanks
Shveta