Thread

Re: Proposal: Conflict log history table for Logical Replication

Dilip Kumar <dilipbalaut@gmail.com> — 2025-12-14T10:21:40Z

On Fri, Dec 12, 2025 at 3:04 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Thu, Dec 11, 2025 at 7:49 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> >
> > I was considering the interdependence between the subscription and the
> > conflict log table (CLT). IMHO, it would be logical to establish the
> > subscription as dependent on the CLT. This way, if someone attempts to
> > drop the CLT, the system would recognize the dependency of the
> > subscription and prevent the drop unless the subscription is removed
> > first or the CASCADE option is used.
> >
> > However, while investigating this, I encountered an error [1] stating
> > that global objects are not supported in this context. This indicates
> > that global objects cannot be made dependent on local objects.
> >
>
> What we need here is an equivalent of DEPENDENCY_INTERNAL for database
> objects. For example, consider following case:
> postgres=# create table t1(c1 int primary key);
> CREATE TABLE
> postgres=# \d+ t1
>                                            Table "public.t1"
>  Column |  Type   | Collation | Nullable | Default | Storage |
> Compression | Stats target | Description
> --------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
>  c1     | integer |           | not null |         | plain   |
>     |              |
> Indexes:
>     "t1_pkey" PRIMARY KEY, btree (c1)
> Publications:
>     "pub1"
> Not-null constraints:
>     "t1_c1_not_null" NOT NULL "c1"
> Access method: heap
> postgres=# drop index t1_pkey;
> ERROR:  cannot drop index t1_pkey because constraint t1_pkey on table
> t1 requires it
> HINT:  You can drop constraint t1_pkey on table t1 instead.
>
> Here, the PK index is created as part for CREATE TABLE operation and
> pk_index is not allowed to be dropped independently.
>
> > Although making an object dependent on global/shared objects is
> > possible for certain types of shared objects [2], this is not our main
> > objective.
> >
>
> As per my understanding from the above example, we need something like
> that only for shared object subscription and (internally created)
> table.

Yeah that seems to be exactly what we want, so I tried doing that by
recording DEPENDENCY_INTERNAL dependency of CLT on subscription[1] and
it is behaving as we want[2].  And while dropping the subscription or
altering CLT we can delete internal dependency so that CLT get dropped
automatically[3]

I will send an updated patch after testing a few more scenarios and
fixing other pending issues.

[1]
+       ObjectAddressSet(myself, RelationRelationId, relid);
+       ObjectAddressSet(subaddr, SubscriptionRelationId, subid);
+       recordDependencyOn(&myself, &subaddr, DEPENDENCY_INTERNAL);


[2]
postgres[670778]=# DROP TABLE myschema.conflict_log_history2;
ERROR:  2BP01: cannot drop table myschema.conflict_log_history2
because subscription sub requires it
HINT:  You can drop subscription sub instead.
LOCATION:  findDependentObjects, dependency.c:788
postgres[670778]=#

[3]
ObjectAddressSet(object, SubscriptionRelationId, subid);
performDeletion(&object, DROP_CASCADE
                           PERFORM_DELETION_INTERNAL |
                           PERFORM_DELETION_SKIP_ORIGINAL);



--
Regards,
Dilip Kumar
Google