Re: Have pg_basebackup write "dbname" in "primary_conninfo"?

On Tue, Feb 20, 2024 at 5:04 AM Ian Lawrence Barwick <barwick(at)gmail(dot)com> wrote:
>
>
> With the addition of "pg_sync_replication_slots()", there is now a use-case for
> including "dbname" in "primary_conninfo" and the docs have changed from
> stating [1]:
>
> Do not specify a database name in the primary_conninfo string.
>
> to [2]:
>
> For replication slot synchronization (see Section 48.2.3), it is also
> necessary to specify a valid dbname in the primary_conninfo string. This will
> only be used for slot synchronization. It is ignored for streaming.
>
> [1] https://www.postgresql.org/docs/16/runtime-config-replication.html#RUNTIME-CONFIG-REPLICATION-STANDBY
> [2] https://www.postgresql.org/docs/devel/runtime-config-replication.html#RUNTIME-CONFIG-REPLICATION-STANDBY
>
> However, when setting up a standby (with the intent of creating a logical
> standby) with pg_basebackup, providing the -R/-write-recovery-conf option
> results in a "primary_conninfo" string being generated without a "dbname"
> parameter, which requires a manual change should one intend to use
> "pg_sync_replication_slots()".
>
> I can't see any reason for continuing to omit "dbname", so suggest it should
> only continue to be omitted for 16 and earlier; see attached patch.
>
> Note that this does mean that if the conninfo string provided to pg_basebackup
> does not contain "dbname", the generated "primary_conninfo" GUC will default to
> "dbname=replication".
>

When I tried, it defaulted to user name:
Default case connection string:
primary_conninfo = 'user=KapilaAm
passfile=''C:\\\\Users\\\\kapilaam\\\\AppData\\\\Roaming/postgresql/pgpass.conf''
channel_binding=disable dbname=KapilaAm port=5432 sslmode=disable
sslcompression=0 sslcertmode=disable sslsni=1
ssl_min_protocol_version=TLSv1.2 gssencmode=disable
krbsrvname=postgres gssdelegation=0 target_session_attrs=any
load_balance_hosts=disable'

When I specified -d "dbname = postgres" during backup:
primary_conninfo = 'user=KapilaAm
passfile=''C:\\\\Users\\\\kapilaam\\\\AppData\\\\Roaming/postgresql/pgpass.conf''
channel_binding=disable dbname=KapilaAm port=5432 sslmode=disable
sslcompression=0 sslcertmode=disable sslsni=1
ssl_min_protocol_version=TLSv1.2 gssencmode=disable
krbsrvname=postgres gssdelegation=0 target_session_attrs=any
load_balance_hosts=disable'

I think it makes sense to include dbname in the connection string
programmatically (with some option) for slot sync functionality but it
is not clear to me if there is any impact of the same when the standby
is not set up to sync up logical slots. I haven't checked but if there
is a way to distinguish the case where we write dbname only when
specified by the user then that would be great but this is worth
considering even without that.

--
With Regards,
Amit Kapila.