Re: [PATCH] Extending FK check skipping on replicas to ADD FK and TRUNCATE

And now it also passes tests.

Still learning about the git way of generating PostgreSQL patches,
that's why there are two separate ones

On Sun, Jul 6, 2025 at 4:30 PM Hannu Krosing <hannuk(at)google(dot)com> wrote:
>
> Managed to send wrong patch earlier, this one actually compiles
>
> On Sun, Jul 6, 2025 at 1:48 PM Hannu Krosing <hannuk(at)google(dot)com> wrote:
> >
> > Here is a rebased patch
> >
> > this time I did not indent the part under
> > if(SessionReplicationRole != SESSION_REPLICATION_ROLE_REPLICA)
> > {
> > ... (<did not add indent>
> > }
> >
> > so it is immediately obviuos from the patch what is added.
> >
> > I can add the indent later, or just let pg_ident take care of this in due time
> >
> > On Sat, May 24, 2025 at 4:02 PM Hannu Krosing <hannuk(at)google(dot)com> wrote:
> > >
> > > I would also argue for treating this as a bug and back-porting to all
> > > supported versions - a quick look at v13 seems to confirm that the
> > > wrapped code has not changed at least since then.
> > >
> > > I don't think we can claim the current state is Working As Intended
> > > unless someone stands up and says they really intended it to work this
> > > way :)
> > >
> > > On Sat, May 24, 2025 at 3:47 PM Hannu Krosing <hannuk(at)google(dot)com> wrote:
> > > >
> > > > Hello Everybody,
> > > >
> > > > Currently setting `session_replication_role` to `replica` disables
> > > > foreign key checks allowing, among other things, free table copy order
> > > > and faster CDC apply in logical replication.
> > > >
> > > > But two other cases of foreign keys are still restricted or blocked
> > > > even with this setting
> > > >
> > > >
> > > > 1. Foreign Key checks during `ALTER TABLE <fkt> ADD FOREIGN KEY
> > > > (<fkcol>) REFERENCES <pkt>(<pkcol>);`
> > > >
> > > > These can be really, Really, REALLY slow when the PK table is huge
> > > > (hundreds of billions of rows). And here I mean taking-tens-of-days
> > > > slow in extreme cases.
> > > >
> > > > And they are also completely unnecessary when the table data comes
> > > > from a known good source.
> > > >
> > > >
> > > > 2. `TRUNCATE <referenced table>` is blocked when there are any foreign
> > > > keys referencing the <referenced table>
> > > >
> > > > But you still can mess up your database in exactly the same way by
> > > > running `DELETE FROM <referenced table>`, just a little (or a lot)
> > > > slower.
> > > >
> > > >
> > > > The attached patch fixes this, allowing both cases to work when `SET
> > > > session_replication_role=replica;` is in force:
> > > >
> > > > ### Test for `ALTER TABLE <fkt> ADD FOREIGN KEY (<fkcol>) REFERENCES
> > > > <pkt>(<pkcol>);`
> > > >
> > > > CREATE TABLE pkt(id int PRIMARY KEY);
> > > > CREATE TABLE fkt(fk int);
> > > > INSERT INTO fkt VALUES(1);
> > > >
> > > > ALTER TABLE fkt ADD FOREIGN KEY (fk) REFERENCES pkt(id); -- fails
> > > >
> > > > SET session_replication_role=replica;
> > > > ALTER TABLE fkt ADD FOREIGN KEY (fk) REFERENCES pkt(id); -- now succeeds
> > > >
> > > > ### Test for `TRUNCATE <referenced table>`
> > > >
> > > > CREATE TABLE pkt(id int PRIMARY KEY);
> > > > CREATE TABLE fkt(fk int REFERENCES pkt(id));
> > > >
> > > > TRUNCATE pkt; -- fails
> > > >
> > > > SET session_replication_role=replica;
> > > > TRUNCATE pkt; -- now succeeds
> > > >
> > > >
> > > >
> > > > The patch just wraps two sections of code in
> > > >
> > > > if (SessionReplicationRole != SESSION_REPLICATION_ROLE_REPLICA) {
> > > > ...
> > > > }
> > > >
> > > > and the rest is added indentation for the wrapped.
> > > >
> > > > Plus I added tests, including the until now missing test for
> > > > explicitly the foreign key checks (which can be argued to already be
> > > > covered by generic trigger tests)
> > > >
> > > >
> > > > I am not sure if we need to add anything to current documentation[*]
> > > > which says just this about foreign keys and
> > > > `session_replication_role=replica`
> > > >
> > > > > Since foreign keys are implemented as triggers, setting this parameter to replica also disables all foreign key checks, which can leave data in an inconsistent state if improperly used.
> > > >
> > > > [*] https://www.postgresql.org/docs/17/runtime-config-client.html#GUC-SESSION-REPLICATION-ROLE
> > > >
> > > > Any comments and suggestions are welcome