Force pg_upgrade's to preserve pg_class.oid, not pg_class.relfilenode.

Toast tables have identical pg_class.oid and pg_class.relfilenode, but
for clarity it is good to preserve the pg_class.oid.

Update comments regarding what is preserved, and do some
variable/function renaming for clarity.

1 files changed, 21 insertions, 14 deletions

diff --git a/contrib/pg_upgrade/pg_upgrade.c b/contrib/pg_upgrade/pg_upgrade.c
index 820445ce557..3de47fc62ec 100644
--- a/contrib/pg_upgrade/pg_upgrade.c
+++ b/contrib/pg_upgrade/pg_upgrade.c
@@ -8,23 +8,30 @@
  */
 
 /*
- *	To simplify the upgrade process, we force certain system items to be
- *	consistent between old and new clusters:
+ *	To simplify the upgrade process, we force certain system values to be
+ *	identical between old and new clusters:
  *
- *	We control all assignments of pg_class.relfilenode so we can keep the
- *	same relfilenodes for old and new files.  The only exception is
- *	pg_largeobject, pg_largeobject_metadata, and its indexes, which can
- *	change due to a cluster, reindex, or vacuum full.  (We don't create
- *	those so have no control over their oid/relfilenode values.)
+ *	We control all assignments of pg_class.oid (and relfilenode) so toast
+ *	oids are the same between old and new clusters.  This is important
+ *	because toast oids are stored as toast pointers in user tables.
  *
- *	While pg_class.oid and pg_class.relfilenode are intially the same, they
- *	can diverge due to cluster, reindex, or vacuum full.  The new cluster
- *	will again have matching pg_class.relfilenode and pg_class.oid values,
- *	but based on the new relfilenode value, so the old/new oids might
- *	differ.
+ *	The only place where old/new relfilenode might not match is
+ *	pg_largeobject, pg_largeobject_metadata, and its indexes,
+ *	which can change their relfilenode values due to a cluster, reindex,
+ *	or vacuum full.  (We don't create those so have no control over their
+ *	new relfilenode values.)
  *
- *	We control all assignments of pg_type.oid because these are stored
- *	in composite types.
+ *	FYI, while pg_class.oid and pg_class.relfilenode are intially the same
+ *	in a cluster, but they can diverge due to cluster, reindex, or vacuum
+ *	full.  The new cluster will again have matching pg_class.relfilenode
+ *	and pg_class.oid values, but based on the old relfilenode value, so the
+ *	old/new oids might differ.
+ *
+ *	We control all assignments of pg_type.oid because these oid are stored
+ *	in user composite type values.
+ *
+ *	We control all assignments of pg_enum.oid because these oid are stored
+ *	in user tables as enum values.
  */