Robert Haas wrote:
> The transforms patch seems to have forgotten to add
> TransformRelationId to object_classes[], much like the RLS patch
> forgot to add PolicyRelationId in the same place.
> 
> Fixing this is easy, but ISTM that we need to insert some sort of a
> guard to prevent people from continuing to forget this, because it's
> apparently quite easy to do.  Perhaps add_object_address should
> Assert(OidIsValid(object_classes[oclass])),

The problem is that there aren't enough callers of add_object_address:
there are many indexes of that array that aren't ever accessed and so
it's not obvious when the array is broken.  If we were to put
OCLASS_CLASS at the end instead of at the beginning, that would fix the
problem by making it immediately obvious when things get broken this
way, because the value used in the most common case would shift around
every time we add another value.  (Of course, we'd have to instruct
people to not add new members after the pg_class entry.)

I just tried this, and it's a bit nasty: while it does causes the tests
to fail, it's not at all obvious that there's a connection between the
failure and object_classes[].  I think we can solve that by adding a
comment to ObjectClass.  Here's the first hunk in the large regression
failure:

*** /pgsql/source/master/src/test/regress/expected/triggers.out 2015-05-22 
20:09:28.936186873 +0200
--- 
/home/alvherre/Code/pgsql/build/master/src/test/regress/results/triggers.out    
2015-07-18 17:26:13.
664764070 +0200
***************
*** 1429,1437 ****
  (4 rows)
  
  DROP TABLE city_table CASCADE;
- NOTICE:  drop cascades to 2 other objects
- DETAIL:  drop cascades to view city_view
- drop cascades to view european_city_view
  DROP TABLE country_table;
  -- Test pg_trigger_depth()
  create table depth_a (id int not null primary key);
--- 1429,1434 ----

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
diff --git a/src/backend/catalog/dependency.c b/src/backend/catalog/dependency.c
index c1212e9..0107c53 100644
--- a/src/backend/catalog/dependency.c
+++ b/src/backend/catalog/dependency.c
@@ -127,7 +127,6 @@ typedef struct
  * See also getObjectClass().
  */
 static const Oid object_classes[MAX_OCLASS] = {
-	RelationRelationId,			/* OCLASS_CLASS */
 	ProcedureRelationId,		/* OCLASS_PROC */
 	TypeRelationId,				/* OCLASS_TYPE */
 	CastRelationId,				/* OCLASS_CAST */
@@ -158,7 +157,9 @@ static const Oid object_classes[MAX_OCLASS] = {
 	DefaultAclRelationId,		/* OCLASS_DEFACL */
 	ExtensionRelationId,		/* OCLASS_EXTENSION */
 	EventTriggerRelationId,		/* OCLASS_EVENT_TRIGGER */
-	PolicyRelationId			/* OCLASS_POLICY */
+	PolicyRelationId,			/* OCLASS_POLICY */
+	TransformRelationId,		/* OCLASS_POLICY */
+	RelationRelationId			/* OCLASS_CLASS */
 };
 
 
diff --git a/src/include/catalog/dependency.h b/src/include/catalog/dependency.h
index 5da18c2..6f4802d 100644
--- a/src/include/catalog/dependency.h
+++ b/src/include/catalog/dependency.h
@@ -112,11 +112,10 @@ typedef struct ObjectAddresses ObjectAddresses;
 
 /*
  * This enum covers all system catalogs whose OIDs can appear in
- * pg_depend.classId or pg_shdepend.classId.
+ * pg_depend.classId or pg_shdepend.classId.  See also object_classes[].
  */
 typedef enum ObjectClass
 {
-	OCLASS_CLASS,				/* pg_class */
 	OCLASS_PROC,				/* pg_proc */
 	OCLASS_TYPE,				/* pg_type */
 	OCLASS_CAST,				/* pg_cast */
@@ -149,6 +148,11 @@ typedef enum ObjectClass
 	OCLASS_EVENT_TRIGGER,		/* pg_event_trigger */
 	OCLASS_POLICY,				/* pg_policy */
 	OCLASS_TRANSFORM,			/* pg_transform */
+	/*
+	 * Keep this previous-to-last, see
+	 * https://www.postgresql.org/message-id/
+	 */
+	OCLASS_CLASS,				/* pg_class */
 	MAX_OCLASS					/* MUST BE LAST */
 } ObjectClass;
 
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