Refuse ATTACH of a table referenced by a foreign key

Trying to attach a table as a partition which is already on the
referenced side of a foreign key on the partitioned table that it is
being attached to, leads to strange behavior: we try to clone the
foreign key from the parent to the partition, but this new FK points to
the partition itself, and the mix of pg_constraint rows and triggers
doesn't behave well.

Rather than trying to untangle the mess (which might be possible given
sufficient time), I opted to forbid the ATTACH.  This doesn't seem a
problematic restriction, given that we already fail to create the
foreign key if you do it the other way around, that is, having the
partition first and the FK second.

Backpatch to all supported branches.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/18541-628a61bc267cd2d3@postgresql.org

src/backend/commands/tablecmds.c

@@ -10318,6 +10318,23 @@ CloneFkReferencing(List **wqueue, Relation parentRel, Relation partRel)
 	{
 		ForeignKeyCacheInfo *fk = lfirst(cell);
 
+		/*
+		 * Refuse to attach a table as partition that this partitioned table
+		 * already has a foreign key to.  This isn't useful schema, which is
+		 * proven by the fact that there have been no user complaints that
+		 * it's already impossible to achieve this in the opposite direction,
+		 * i.e., creating a foreign key that references a partition.  This
+		 * restriction allows us to dodge some complexities around
+		 * pg_constraint and pg_trigger row creations that would be needed
+		 * during ATTACH/DETACH for this kind of relationship.
+		 */
+		if (fk->confrelid == RelationGetRelid(partRel))
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("can't attach table \"%s\" as a partition which is referenced by foreign key \"%s\"",
+							RelationGetRelationName(partRel),
+							get_constraint_name(fk->conoid))));
+
 		clone = lappend_oid(clone, fk->conoid);
 	}
 

src/test/regress/expected/foreign_key.out

@@ -1967,6 +1967,23 @@ INSERT INTO fk_notpartitioned_pk VALUES (1600, 601), (1600, 1601);
 ALTER TABLE fk_partitioned_fk ATTACH PARTITION fk_partitioned_fk_2
   FOR VALUES IN (1600);
 -- leave these tables around intentionally
+-- Verify that attaching a table that's referenced by an existing FK
+-- in the parent throws an error
+CREATE TABLE fk_partitioned_pk_6 (a int PRIMARY KEY);
+CREATE TABLE fk_partitioned_fk_6 (a int REFERENCES fk_partitioned_pk_6) PARTITION BY LIST (a);
+ALTER TABLE fk_partitioned_fk_6 ATTACH PARTITION fk_partitioned_pk_6 FOR VALUES IN (1);
+ERROR:  can't attach table "fk_partitioned_pk_6" as a partition which is referenced by foreign key "fk_partitioned_fk_6_a_fkey"
+DROP TABLE fk_partitioned_pk_6, fk_partitioned_fk_6;
+-- This case is similar to above, but the referenced relation is one level
+-- lower in the hierarchy.  This one fails in a different way as the above,
+-- because we don't bother to protect against this case explicitly.  If the
+-- current error stops happening, we'll need to add a better protection.
+CREATE TABLE fk_partitioned_pk_6 (a int PRIMARY KEY) PARTITION BY list (a);
+CREATE TABLE fk_partitioned_pk_61 PARTITION OF fk_partitioned_pk_6 FOR VALUES IN (1);
+CREATE TABLE fk_partitioned_fk_6 (a int REFERENCES fk_partitioned_pk_61) PARTITION BY LIST (a);
+ALTER TABLE fk_partitioned_fk_6 ATTACH PARTITION fk_partitioned_pk_6 FOR VALUES IN (1);
+ERROR:  cannot ALTER TABLE "fk_partitioned_pk_61" because it is being used by active queries in this session
+DROP TABLE fk_partitioned_pk_6, fk_partitioned_fk_6;
 -- test the case when the referenced table is owned by a different user
 create role regress_other_partitioned_fk_owner;
 grant references on fk_notpartitioned_pk to regress_other_partitioned_fk_owner;

src/test/regress/sql/foreign_key.sql

@@ -1417,6 +1417,23 @@ ALTER TABLE fk_partitioned_fk ATTACH PARTITION fk_partitioned_fk_2
 
 -- leave these tables around intentionally
 
+-- Verify that attaching a table that's referenced by an existing FK
+-- in the parent throws an error
+CREATE TABLE fk_partitioned_pk_6 (a int PRIMARY KEY);
+CREATE TABLE fk_partitioned_fk_6 (a int REFERENCES fk_partitioned_pk_6) PARTITION BY LIST (a);
+ALTER TABLE fk_partitioned_fk_6 ATTACH PARTITION fk_partitioned_pk_6 FOR VALUES IN (1);
+DROP TABLE fk_partitioned_pk_6, fk_partitioned_fk_6;
+
+-- This case is similar to above, but the referenced relation is one level
+-- lower in the hierarchy.  This one fails in a different way as the above,
+-- because we don't bother to protect against this case explicitly.  If the
+-- current error stops happening, we'll need to add a better protection.
+CREATE TABLE fk_partitioned_pk_6 (a int PRIMARY KEY) PARTITION BY list (a);
+CREATE TABLE fk_partitioned_pk_61 PARTITION OF fk_partitioned_pk_6 FOR VALUES IN (1);
+CREATE TABLE fk_partitioned_fk_6 (a int REFERENCES fk_partitioned_pk_61) PARTITION BY LIST (a);
+ALTER TABLE fk_partitioned_fk_6 ATTACH PARTITION fk_partitioned_pk_6 FOR VALUES IN (1);
+DROP TABLE fk_partitioned_pk_6, fk_partitioned_fk_6;
+
 -- test the case when the referenced table is owned by a different user
 create role regress_other_partitioned_fk_owner;
 grant references on fk_notpartitioned_pk to regress_other_partitioned_fk_owner;