Function GetParentedForeignKeyRefs

src/backend/commands/tablecmds.c:23026–23072

* Return an OID list of constraints that reference the given relation * that are marked as having a parent constraints. */

 * that are marked as having a parent constraints.
 */
static List *
GetParentedForeignKeyRefs(Relation partition)
{
	Relation	pg_constraint;
	HeapTuple	tuple;
	SysScanDesc scan;
	ScanKeyData key[2];
	List	   *constraints = NIL;

	/*
	 * If no indexes, or no columns are referenceable by FKs, we can avoid the
	 * scan.
	 */
	if (RelationGetIndexList(partition) == NIL ||
		bms_is_empty(RelationGetIndexAttrBitmap(partition,
												INDEX_ATTR_BITMAP_KEY)))
		return NIL;

	/* Search for constraints referencing this table */
	pg_constraint = table_open(ConstraintRelationId, AccessShareLock);
	ScanKeyInit(&key[0],
				Anum_pg_constraint_confrelid, BTEqualStrategyNumber,
				F_OIDEQ, ObjectIdGetDatum(RelationGetRelid(partition)));
	ScanKeyInit(&key[1],
				Anum_pg_constraint_contype, BTEqualStrategyNumber,
				F_CHAREQ, CharGetDatum(CONSTRAINT_FOREIGN));

	/* XXX This is a seqscan, as we don't have a usable index */
	scan = systable_beginscan(pg_constraint, InvalidOid, true, NULL, 2, key);
	while ((tuple = systable_getnext(scan)) != NULL)
	{
		Form_pg_constraint constrForm = (Form_pg_constraint) GETSTRUCT(tuple);

		/*
		 * We only need to process constraints that are part of larger ones.
		 */
		if (!OidIsValid(constrForm->conparentid))
			continue;

		constraints = lappend_oid(constraints, constrForm->oid);
	}

	systable_endscan(scan);
	table_close(pg_constraint, AccessShareLock);

	return constraints;
}

/*
 * During DETACH PARTITION, verify that any foreign keys pointing to the