Re: Improve logical replication usability when tables lack primary keys

> On Dec 18, 2025, at 22:49, Euler Taveira <euler(at)eulerto(dot)com> wrote:
>
> On Wed, Dec 17, 2025, at 6:43 PM, GRANT ZHOU wrote:
>> On Wed, Dec 17, 2025 at 12:50 PM Euler Taveira <euler(at)eulerto(dot)com> wrote:
>>> Each table needs to say what's its row identifier. The user created a table
>>> without primary key. Well, create a primary key. There are dozens of thousands
>>> of objects. Use a script.
>> However, I’d like to share a user perspective regarding the "use a
>> script" approach. The main value of `FOR TABLES IN SCHEMA` is
>> *in-database automation*. If users still need to maintain external
>> scripts to monitor and `ALTER` new tables to prevent replication
>> errors, it significantly diminishes the value of that automation.
>>
>
> As I tried to explain in the previous email, the problem with FOR ALL TABLES
> and FOR TABLES IN SCHEMA syntax is that the is no catalog information about the
> relations; the list of relations is collected at runtime.
>
> When I suggested "use a script" I was referring to fix the logical replication
> setup regarding the lack of primary key. There is no need to have an automation
> outside the database, use an event trigger. If your lazy user doesn't create
> the primary key, assign REPLICA IDENTITY FULL. Something like
>
> -- This example is far from being a complete solution for fixing the lack of
> -- primary key in a logical replication scenario.
> -- ALTER TABLE should be supported too
> CREATE OR REPLACE FUNCTION event_trigger_for_replica_identity()
> RETURNS event_trigger LANGUAGE plpgsql AS $$
> DECLARE
> obj record;
> rec record;
> ricnt integer := 0;
> BEGIN
> FOR obj IN SELECT * FROM pg_event_trigger_ddl_commands()
> LOOP
> IF obj.command_tag = 'CREATE TABLE' THEN
> SELECT COUNT(*) INTO ricnt FROM pg_index WHERE indrelid = obj.objid AND indisprimary;
> RAISE NOTICE 'ricnt: %', ricnt;
> IF ricnt = 0 THEN
> EXECUTE 'ALTER TABLE ' || obj.object_identity || ' REPLICA IDENTITY FULL';
> END IF;
> END IF;
> END LOOP;
> END;
> $$;
>
> CREATE EVENT TRIGGER event_trigger_for_replica_identity
> ON ddl_command_end
> EXECUTE FUNCTION event_trigger_for_replica_identity();
>
> CREATE TABLE event_trigger_test_1 (a int);
> \d+ event_trigger_test_1
> CREATE TABLE event_trigger_test_2 (a int primary key);
> \d+ event_trigger_test_2
> CREATE TABLE event_trigger_test_3 (a int, b text not null, primary key(b));
> \d+ event_trigger_test_3
> --ALTER TABLE event_trigger_test_3 DROP CONSTRAINT event_trigger_test_3_pkey;
> --\d+ event_trigger_test_3
>
> DROP EVENT TRIGGER event_trigger_for_replica_identity;
> DROP FUNCTION event_trigger_for_replica_identity;
> DROP TABLE event_trigger_test_1, event_trigger_test_2, event_trigger_test_3;
>
> 8<----------------------------------------------------------------------------8<
>
>> Additionally, tables without Primary Keys are valid SQL and extremely
>> common in enterprise environments (e.g., audit logs, data warehousing).
>> In large-scale deployments, enforcing PKs on every single table isn't
>> always practical.
>>
>
> I'm not saying users shouldn't create tables without a primary key. I'm arguing
> that this decision should take into account what adjustments need to be made to
> use these tables in logical replication.
>
>>
>> I think the goal of this proposal is not to change the underlying table
>> property design, but rather to seek a mechanism (like a Publication
>> option) to ensure this automation functions safely without external
>> intervention. It is simply about allowing the database to handle these
>> valid, common scenarios gracefully when automation is enabled.
>>
>
> You didn't get it. You already have one property to handle it and you are
> proposing to add a second property to handle it.
>
> I think you are pursuing the wrong solution. IMO we need a solution to enforce
> that the logical replication contract is valid. If you create or modify a table
> that is part of a publication, there is no validation that that table complies
> with the publication properties (update and delete properties should require an
> appropriate replica identity). We should close the gaps in both publication and
> table.
>

If I summarize Euler’s position in short words: discipline over convenience. I actually strongly agree with that. In PG we generally prefer explicit over implicit behavior, and predictability over magic.

Based on the discussion so far, I think we share the following design goals:

1) Keep replica identity as a table property.
2) Avoid silent runtime failures when FOR TABLES IN SCHEMA pulls in tables without primary keys.
3) Avoid global or implicit behavior changes.
4) Preserve explicit opt-in for higher WAL cost.
5) Keep the logical replication contract explicit and enforceable.

I’ve been thinking about whether adding a new replica identity could meet these goals.

Today we have four replica identities: DEFAULT (PK, fallback to NONE), INDEX, FULL, and NONE.

What if we introduce a new replica identity, tentatively called “FORCE”: PK with fallback to FULL. (Let’s keep our focus on the design, not argue the name for now.)

With this approach:

1) Replica identity remains a table property.
2) Publication membership is still evaluated at runtime, so FOR TABLES IN SCHEMA is not special-cased.
3) No new GUCs are required.
4) The user must explicitly opt in by setting the replica identity. Once FORCE is chosen, adding or dropping a primary key later does not silently break UPDATE/DELETE replication.
5) The logical replication contract remains explicit; the table declares that it is safe for UPDATE/DELETE replication even without a PK, at the cost of higher WAL volume.

This feels like a small, explicit extension of the existing RI semantics. Notably, REPLICA IDENTITY DEFAULT already has conditional behavior (PK fallback
to NONE), so conditional RI behavior is not new, this would just make a different fallback explicit and user-chosen.

After that, we could consider a database-level default_replica_identity setting, applied at table creation time, for environments that want this behavior consistently. But that would only make sense if we first agree on the table-level mechanism.

I’m interested in whether this direction aligns better with the goals above.

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