Un-break pg_upgrade from pre-v12 servers.

I neglected to test this scenario while preparing commit f3faf35f,
so of course it was broken, thanks to some very obscure and undocumented
code in pg_dump.  Pre-v12 databases might have toast tables attached to
partitioned tables, which we need to ignore since newer servers never
create such useless toast tables.  There was a filter for this case in
binary_upgrade_set_type_oids_by_rel_oid(), which appeared to just
prevent the pg_type OID from being copied.  But actually it managed to
prevent the toast table from being created at all --- or it did before
I took out that logic.  But that was a fundamentally bizarre place to be
making the test in the first place.  The place where the filter should
have been, one would think, is binary_upgrade_set_pg_class_oids(), so
add it there.

While at it, reorganize binary_upgrade_set_pg_class_oids() so that it
doesn't make a completely useless query when it knows it's being
invoked for an index.  And correct a comment that mis-described the
scenario where we need to force creation of a TOAST table.

Per buildfarm.

src/bin/pg_dump/pg_dump.c

@@ -4525,43 +4525,56 @@ binary_upgrade_set_pg_class_oids(Archive *fout,
 								 PQExpBuffer upgrade_buffer, Oid pg_class_oid,
 								 bool is_index)
 {
-	PQExpBuffer upgrade_query = createPQExpBuffer();
-	PGresult   *upgrade_res;
-	Oid			pg_class_reltoastrelid;
-	Oid			pg_index_indexrelid;
-
-	appendPQExpBuffer(upgrade_query,
-					  "SELECT c.reltoastrelid, i.indexrelid "
-					  "FROM pg_catalog.pg_class c LEFT JOIN "
-					  "pg_catalog.pg_index i ON (c.reltoastrelid = i.indrelid AND i.indisvalid) "
-					  "WHERE c.oid = '%u'::pg_catalog.oid;",
-					  pg_class_oid);
-
-	upgrade_res = ExecuteSqlQueryForSingleRow(fout, upgrade_query->data);
-
-	pg_class_reltoastrelid = atooid(PQgetvalue(upgrade_res, 0, PQfnumber(upgrade_res, "reltoastrelid")));
-	pg_index_indexrelid = atooid(PQgetvalue(upgrade_res, 0, PQfnumber(upgrade_res, "indexrelid")));
-
 	appendPQExpBufferStr(upgrade_buffer,
 						 "\n-- For binary upgrade, must preserve pg_class oids\n");
 
 	if (!is_index)
 	{
+		PQExpBuffer upgrade_query = createPQExpBuffer();
+		PGresult   *upgrade_res;
+		Oid			pg_class_reltoastrelid;
+		char		pg_class_relkind;
+		Oid			pg_index_indexrelid;
+
 		appendPQExpBuffer(upgrade_buffer,
 						  "SELECT pg_catalog.binary_upgrade_set_next_heap_pg_class_oid('%u'::pg_catalog.oid);\n",
 						  pg_class_oid);
-		/* only tables have toast tables, not indexes */
-		if (OidIsValid(pg_class_reltoastrelid))
-		{
-			/*
-			 * One complexity is that the table definition might not require
-			 * the creation of a TOAST table, and the TOAST table might have
-			 * been created long after table creation, when the table was
-			 * loaded with wide data.  By setting the TOAST oid we force
-			 * creation of the TOAST heap and TOAST index by the backend so we
-			 * can cleanly copy the files during binary upgrade.
-			 */
 
+		/*
+		 * Preserve the OIDs of the table's toast table and index, if any.
+		 * Indexes cannot have toast tables, so we need not make this probe in
+		 * the index code path.
+		 *
+		 * One complexity is that the current table definition might not
+		 * require the creation of a TOAST table, but the old database might
+		 * have a TOAST table that was created earlier, before some wide
+		 * columns were dropped.  By setting the TOAST oid we force creation
+		 * of the TOAST heap and index by the new backend, so we can copy the
+		 * files during binary upgrade without worrying about this case.
+		 */
+		appendPQExpBuffer(upgrade_query,
+						  "SELECT c.reltoastrelid, c.relkind, i.indexrelid "
+						  "FROM pg_catalog.pg_class c LEFT JOIN "
+						  "pg_catalog.pg_index i ON (c.reltoastrelid = i.indrelid AND i.indisvalid) "
+						  "WHERE c.oid = '%u'::pg_catalog.oid;",
+						  pg_class_oid);
+
+		upgrade_res = ExecuteSqlQueryForSingleRow(fout, upgrade_query->data);
+
+		pg_class_reltoastrelid = atooid(PQgetvalue(upgrade_res, 0,
+												   PQfnumber(upgrade_res, "reltoastrelid")));
+		pg_class_relkind = *PQgetvalue(upgrade_res, 0,
+									   PQfnumber(upgrade_res, "relkind"));
+		pg_index_indexrelid = atooid(PQgetvalue(upgrade_res, 0,
+												PQfnumber(upgrade_res, "indexrelid")));
+
+		/*
+		 * In a pre-v12 database, partitioned tables might be marked as having
+		 * toast tables, but we should ignore them if so.
+		 */
+		if (OidIsValid(pg_class_reltoastrelid) &&
+			pg_class_relkind != RELKIND_PARTITIONED_TABLE)
+		{
 			appendPQExpBuffer(upgrade_buffer,
 							  "SELECT pg_catalog.binary_upgrade_set_next_toast_pg_class_oid('%u'::pg_catalog.oid);\n",
 							  pg_class_reltoastrelid);
@@ -4571,6 +4584,9 @@ binary_upgrade_set_pg_class_oids(Archive *fout,
 							  "SELECT pg_catalog.binary_upgrade_set_next_index_pg_class_oid('%u'::pg_catalog.oid);\n",
 							  pg_index_indexrelid);
 		}
+
+		PQclear(upgrade_res);
+		destroyPQExpBuffer(upgrade_query);
 	}
 	else
 		appendPQExpBuffer(upgrade_buffer,
@@ -4578,9 +4594,6 @@ binary_upgrade_set_pg_class_oids(Archive *fout,
 						  pg_class_oid);
 
 	appendPQExpBufferChar(upgrade_buffer, '\n');
-
-	PQclear(upgrade_res);
-	destroyPQExpBuffer(upgrade_query);
 }
 
 /*