Fix some comments referring to past features

Timestamp can only be an int64 since b9d092c, and support for WITH OIDS
has been removed as of 578b229.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20200612023709.GC14879@telsasoft.com

src/backend/access/common/tupdesc.c

@@ -786,9 +786,7 @@ TupleDescInitEntryCollation(TupleDesc desc,
  *
  * Given a relation schema (list of ColumnDef nodes), build a TupleDesc.
  *
- * Note: the default assumption is no OIDs; caller may modify the returned
- * TupleDesc if it wants OIDs.  Also, tdtypeid will need to be filled in
- * later on.
+ * Note: tdtypeid will need to be filled in later on.
  */
 TupleDesc
 BuildDescForRelation(List *schema)

src/backend/commands/tablecmds.c

@@ -4829,12 +4829,11 @@ ATRewriteTables(AlterTableStmt *parsetree, List **wqueue, LOCKMODE lockmode,
 			continue;
 
 		/*
-		 * If we change column data types or add/remove OIDs, the operation
-		 * has to be propagated to tables that use this table's rowtype as a
-		 * column type.  tab->newvals will also be non-NULL in the case where
-		 * we're adding a column with a default.  We choose to forbid that
-		 * case as well, since composite types might eventually support
-		 * defaults.
+		 * If we change column data types, the operation has to be propagated
+		 * to tables that use this table's rowtype as a column type.
+		 * tab->newvals will also be non-NULL in the case where we're adding a
+		 * column with a default.  We choose to forbid that case as well,
+		 * since composite types might eventually support defaults.
 		 *
 		 * (Eventually we'll probably need to check for composite type
 		 * dependencies even when we're just scanning the table without a
@@ -4853,8 +4852,7 @@ ATRewriteTables(AlterTableStmt *parsetree, List **wqueue, LOCKMODE lockmode,
 
 		/*
 		 * We only need to rewrite the table if at least one column needs to
-		 * be recomputed, we are adding/removing the OID column, or we are
-		 * changing its persistence.
+		 * be recomputed, or we are changing its persistence.
 		 *
 		 * There are two reasons for requiring a rewrite when changing
 		 * persistence: on one hand, we need to ensure that the buffers

src/backend/utils/adt/selfuncs.c

@@ -4018,8 +4018,8 @@ estimate_multivariate_ndistinct(PlannerInfo *root, RelOptInfo *rel,
  * to be treated separately.
  *
  * The several datatypes representing absolute times are all converted
- * to Timestamp, which is actually a double, and then we just use that
- * double value.  Note this will give correct results even for the "special"
+ * to Timestamp, which is actually an int64, and then we promote that to
+ * a double.  Note this will give correct results even for the "special"
  * values of Timestamp, since those are chosen to compare correctly;
  * see timestamp_cmp.
  *