C comments: improve description of relfilenode uniqueness

Report by Antonin Houska

src/backend/catalog/catalog.c

@@ -408,8 +408,8 @@ GetNewOidWithIndex(Relation relation, Oid indexId, AttrNumber oidcolumn)
 
 /*
  * GetNewRelFileNode
- *		Generate a new relfilenode number that is unique within the given
- *		tablespace.
+ *		Generate a new relfilenode number that is unique within the
+ *		database of the given tablespace.
  *
  * If the relfilenode will also be used as the relation's OID, pass the
  * opened pg_class catalog, and this routine will guarantee that the result

src/backend/commands/tablecmds.c

@@ -9078,8 +9078,8 @@ ATExecSetTableSpace(Oid tableOid, Oid newTableSpace, LOCKMODE lockmode)
 	FlushRelationBuffers(rel);
 
 	/*
-	 * Relfilenodes are not unique across tablespaces, so we need to allocate
-	 * a new one in the new tablespace.
+	 * Relfilenodes are not unique in databases across tablespaces, so we
+	 * need to allocate a new one in the new tablespace.
 	 */
 	newrelfilenode = GetNewRelFileNode(newTableSpace, NULL,
 									   rel->rd_rel->relpersistence);

src/include/storage/relfilenode.h

@@ -55,7 +55,8 @@ typedef enum ForkNumber
  * relNode identifies the specific relation.  relNode corresponds to
  * pg_class.relfilenode (NOT pg_class.oid, because we need to be able
  * to assign new physical files to relations in some situations).
- * Notice that relNode is only unique within a particular tablespace.
+ * Notice that relNode is only unique within a database in a particular
+ * tablespace.
  *
  * Note: spcNode must be GLOBALTABLESPACE_OID if and only if dbNode is
  * zero.  We support shared relations only in the "global" tablespace.