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Date: Tue, 11 Nov 2025 09:09:02 -0300
From: "Euler Taveira" <euler@eulerto.com>
To: "Chao Li" <li.evan.chao@gmail.com>,
 "Postgres hackers" <pgsql-hackers@lists.postgresql.org>
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Subject: Re: Improve logical replication usability when tables lack primary
 keys
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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.


-- 
Euler Taveira
EDB   https://www.enterprisedb.com/