Re: Improve logical replication usability when tables lack primary keys - Mailing list pgsql-hackers

> On Nov 11, 2025, at 20:09, Euler Taveira <euler@eulerto.com> wrote:
>
> On Mon, Nov 10, 2025, at 5:06 AM, Chao Li wrote:
>> I evaluated a few approaches and am proposing the following:
>>
>> - Introduce a new GUC: `logical_replication_fallback_to_full_identity`.
>> - When enabled, if a table being logically replicated has no primary
>> key, the system automatically uses `REPLICATION IDENTITY FULL` for that
>> table.
>> - This setting can be applied at the database level, so large systems
>> do not need to enable it cluster-wide unless desired.
>> - When the WAL sender transmits relation metadata, if fallback has
>> occurred, it explicitly reports `FULL` as the replication identity to
>> the subscriber, so there is limited impact on the subscriber.
>>
>
> If I understand your proposal correctly, you want to add a new fallback to
> replica identity. We already have a fallback for DEFAULT that means no primary
> key is the same as NOTHING. I didn't like your proposal. It is too restrictive.
>
> However, I see some usefulness in introducing a GUC default_replica_identity.
> The proposal is similar to access method (default_table_access_method). The
> DEFAULT option selects the replica identity sets as default_replica_identity
> parameter. You need to add a new option (PRIMARY KEY); that should be the
> default value. (If we don't want to break the backward compatibility, this new
> option should fallback to NOTHING if there is no primary key. Another
> alternative is to have a strict and non-strict option. I prefer the former.) Of
> course, the USING INDEX option cannot be used. For pg_dump, you need to use SET
> command to inform the default_replica_identity value so tables with the same
> option as default_replica_identity doesn't emit an ALTER TABLE command.
>

I’ve thought this over and discussed it with our field teams. It looks to us that introducing a new GUC like
default_replica_identitydoes not really address our pain point. 

Our core requirement is to allow tables without a primary key to use FULL as the replica identity, while tables with a
primarykey should continue to use DEFAULT. 

If we add default_replica_identity and set it to FULL, then a newly created table that does have a primary key would
alsoend up using FULL, which is definitely not what we want. 

As you mentioned, PostgreSQL already has a fallback from DEFAULT to NOTHING. What we actually want is the ability to
customizethis fallback, so that users can choose whether DEFAULT falls back to NOTHING or to FULL. Customizing the
fallbackvia a new GUC would also allow field teams to set this option per database. 

If we do want to add default_replica_identity, I think that should be treated as a separate topic. By the way, could
youexplain what use case you have in mind for it? 

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/