Re: Two issues with version checks in CREATE SUBSCRIPTION

Hi Fujii-san,

On Mon, 22 Dec 2025 at 22:57, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>
> Hi,
>
> While looking at subscription-related code, I noticed two issues related to
> version checks.
>
> if (walrcv_server_version(wrconn) < 19000)
> ereport(ERROR,
> errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> errmsg("cannot enable retain_dead_tuples if the
> publisher is running a version earlier than PostgreSQL 19"));
>
> First, in subscriptioncmds.c this check rejects enabling retain_dead_tuples
> when the publisher is running an older version. However, the comparison uses
> 19000 as v19 value. Since server versions are encoded as 190000 for v19,
> this appears to be a typo and allows the option to be enabled unexpectedly
> on pre-v19 publishers. The attached 0001 patch fixes this by correcting
> the version constant.
>
I tested this. I created a publisher in PG17 and tried creating
subscriber in PG19.

Without Patch, I am able to create the subscription with
retain_dead_tuples = true.
postgres=# CREATE SUBSCRIPTION sub1 CONNECTION 'host=localhost
port=5432 dbname=postgres' PUBLICATION pub1 WITH (retain
_dead_tuples=true);
NOTICE: created replication slot "sub1" on publisher
CREATE SUBSCRIPTION

With 0001 Patch, this issue is resolved.
postgres=# CREATE SUBSCRIPTION sub2 CONNECTION 'host=localhost
port=5432 dbname=postgres' PUBLICATION pub1 WITH
(retain_dead_tuples=true);
ERROR: cannot enable retain_dead_tuples if the publisher is running a
version earlier than PostgreSQL 19

> Second, CREATE SUBSCRIPTION with copy_data=true and origin='none' currently
> fails when the publisher is running a version earlier than v19, although
> this combination should be supported. The failure occurs because the command
> issues a query calling pg_get_publication_sequences on the publisher,
> which does not exist before v19. The attached 0002 patch fixes this
> by skipping that query when the publisher runs an older version.
>
I am able to reproduce this scenario.
I created a publisher in PG17 and tried creating subscriber in PG19.

Without Patch we are hitting the following error.
postgres=# CREATE SUBSCRIPTION sub1 CONNECTION 'host=localhost
port=5432 dbname=postgres' PUBLICATION pub1 WITH (origin='none');
ERROR: could not receive list of replicated sequences from the
publisher: ERROR: function pg_get_publication_sequences(name) does
not exist
LINE 3: LATERAL pg_get_publication_sequences(P.pubname) GPS
^
HINT: No function matches the given name and argument types. You
might need to add explicit type casts.

And I confirm that the 0002 patch resolves this.

Thanks,
Shlok Kyal