Ignore attempts to add TOAST table to shared or catalog tables

Running ALTER TABLE on any table will check if a TOAST table needs to be
added.  On shared tables, this would previously fail, thus effectively
disabling ALTER TABLE for those tables.  On (non-shared) system
catalogs, on the other hand, it would add a TOAST table, even though we
don't really want TOAST tables on some system catalogs.  In some cases,
it would also fail with an error "AccessExclusiveLock required to add
toast table.", depending on what locks the ALTER TABLE actions had
already taken.

So instead, just ignore attempts to add TOAST tables to such tables,
outside of bootstrap mode, pretending they don't need one.

This allows running ALTER TABLE on such tables without messing up the
TOAST situation.  Legitimate uses for ALTER TABLE on system catalogs
include setting reloptions (say, fillfactor or autovacuum settings).

(All this still requires allow_system_table_mods, which is independent
of this.)

Discussion: https://www.postgresql.org/message-id/flat/e49f825b-fb25-0bc8-8afc-d5ad895c7975@2ndquadrant.com

src/backend/catalog/toasting.c

@@ -18,6 +18,7 @@
 #include "access/tuptoaster.h"
 #include "access/xact.h"
 #include "catalog/binary_upgrade.h"
+#include "catalog/catalog.h"
 #include "catalog/dependency.h"
 #include "catalog/heap.h"
 #include "catalog/index.h"
@@ -146,21 +147,6 @@ create_toast_table(Relation rel, Oid toastOid, Oid toastIndexOid,
 	ObjectAddress baseobject,
 				toastobject;
 
-	/*
-	 * Toast table is shared if and only if its parent is.
-	 *
-	 * We cannot allow toasting a shared relation after initdb (because
-	 * there's no way to mark it toasted in other databases' pg_class).
-	 */
-	shared_relation = rel->rd_rel->relisshared;
-	if (shared_relation && !IsBootstrapProcessingMode())
-		ereport(ERROR,
-				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-				 errmsg("shared tables cannot be toasted after initdb")));
-
-	/* It's mapped if and only if its parent is, too */
-	mapped_relation = RelationIsMapped(rel);
-
 	/*
 	 * Is it already toasted?
 	 */
@@ -260,6 +246,12 @@ create_toast_table(Relation rel, Oid toastOid, Oid toastIndexOid,
 		binary_upgrade_next_toast_pg_type_oid = InvalidOid;
 	}
 
+	/* Toast table is shared if and only if its parent is. */
+	shared_relation = rel->rd_rel->relisshared;
+
+	/* It's mapped if and only if its parent is, too */
+	mapped_relation = RelationIsMapped(rel);
+
 	toast_relid = heap_create_with_catalog(toast_relname,
 										   namespaceid,
 										   rel->rd_rel->reltablespace,
@@ -398,7 +390,6 @@ create_toast_table(Relation rel, Oid toastOid, Oid toastIndexOid,
  * (1) there are any toastable attributes, and (2) the maximum length
  * of a tuple could exceed TOAST_TUPLE_THRESHOLD.  (We don't want to
  * create a toast table for something like "f1 varchar(20)".)
- * No need to create a TOAST table for partitioned tables.
  */
 static bool
 needs_toast_table(Relation rel)
@@ -410,9 +401,28 @@ needs_toast_table(Relation rel)
 	int32		tuple_length;
 	int			i;
 
+	/*
+	 * No need to create a TOAST table for partitioned tables.
+	 */
 	if (rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
 		return false;
 
+	/*
+	 * We cannot allow toasting a shared relation after initdb (because
+	 * there's no way to mark it toasted in other databases' pg_class).
+	 */
+	if (rel->rd_rel->relisshared && !IsBootstrapProcessingMode())
+		return false;
+
+	/*
+	 * Ignore attempts to create toast tables on catalog tables after initdb.
+	 * Which catalogs get toast tables is explicitly chosen in
+	 * catalog/toasting.h.  (We could get here via some ALTER TABLE command if
+	 * the catalog doesn't have a toast table.)
+	 */
+	if (IsCatalogRelation(rel) && !IsBootstrapProcessingMode())
+		return false;
+
 	tupdesc = rel->rd_att;
 
 	for (i = 0; i < tupdesc->natts; i++)