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

Hi Álvaro,

Can you take a look at this?

This is the one I mentioned to you at the PgConf.dev, allowing the
extension of FK check bypassing from the current "don't check on DML"
to also include TRUNCATE and ADD FOREIGN KEY.

This will especially improve replica setup cases where you would want
to create the PK index and Foreign Keys after the initial copy. This
speeds up the copy in extreme cases where the index presence slows
down the initial data transfer.

---
Best regards
Hannu

On Thu, Aug 6, 2026 at 3:10 PM Hannu Krosing <hannuk(at)google(dot)com> wrote:
>
> Hi Rafia
>
> This is exactly how this is supposed to work.
> It allows the replication setup to avoid expensive checks when the
> user knows that they are not needed.
> It also allows you to mess up.
>
> You can currently get the same result by
>
> CREATE TABLE pkt(id int PRIMARY KEY);
> CREATE TABLE fkt(fk int REFERENCES pkt(id));
> INSERT INTO pkt VALUES(1);
> INSERT INTO fkt VALUES(1); -- no matching pkt row
> SET session_replication_role=replica;
> DELETE FROM pkt;
>
>
>
> On Mon, Jul 20, 2026 at 1:18 PM Rafia Sabih <rafia(dot)pghackers(at)gmail(dot)com> wrote:
> >
> >
> >
> > On Sun, 11 Jan 2026 at 14:29, Hannu Krosing <hannuk(at)google(dot)com> wrote:
> >>
> >> rebased to latest
> >>
> >> On Sun, Jul 6, 2025 at 4:48 PM Hannu Krosing <hannuk(at)google(dot)com> wrote:
> >>>
> >>> 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
> >
> >
> > I looked into this patch and found a case which doesn't look right,
> > CREATE TABLE pkt(id int PRIMARY KEY);
> > CREATE TABLE fkt(fk int);
> > INSERT INTO fkt VALUES(1); -- no matching pkt row
> > SET session_replication_role=replica;
> > ALTER TABLE fkt ADD FOREIGN KEY (fk) REFERENCES pkt(id);
> > The Alter command goes through successfully. So, basically, fkt has one row which doesn't follow the constraint.
> >
> > --
> > Regards,
> > Rafia Sabih
> > CYBERTEC PostgreSQL International GmbH