diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index da63b347e58..039b279f786 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -15814,8 +15814,8 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
    <para>
     <function>pg_filenode_relation</> is the reverse of
     <function>pg_relation_filenode</>. Given a <quote>tablespace</> OID and
-    a <quote>filenode</> it returns the associated relation. The default
-    tablespace can be specified as 0.
+	a <quote>filenode</>, it returns the associated relation's OID. For a table
+    in the database's default tablespace, the tablespace can be specified as 0.
    </para>
 
   </sect2>
diff --git a/src/backend/utils/adt/dbsize.c b/src/backend/utils/adt/dbsize.c
index 21d1c946abe..868474680df 100644
--- a/src/backend/utils/adt/dbsize.c
+++ b/src/backend/utils/adt/dbsize.c
@@ -760,13 +760,14 @@ pg_relation_filenode(PG_FUNCTION_ARGS)
  * Get the relation via (reltablespace, relfilenode)
  *
  * This is expected to be used when somebody wants to match an individual file
- * on the filesystem back to its table. Thats not trivially possible via
- * pg_class because that doesn't contain the relfilenodes of shared and nailed
+ * on the filesystem back to its table. That's not trivially possible via
+ * pg_class, because that doesn't contain the relfilenodes of shared and nailed
  * tables.
  *
  * We don't fail but return NULL if we cannot find a mapping.
  *
- * Instead of knowing DEFAULTTABLESPACE_OID you can pass 0.
+ * InvalidOid can be passed instead of the current database's default
+ * tablespace.
  */
 Datum
 pg_filenode_relation(PG_FUNCTION_ARGS)
diff --git a/src/backend/utils/cache/relfilenodemap.c b/src/backend/utils/cache/relfilenodemap.c
index 372cb33c003..2a8f83775d2 100644
--- a/src/backend/utils/cache/relfilenodemap.c
+++ b/src/backend/utils/cache/relfilenodemap.c
@@ -210,7 +210,7 @@ RelidByRelfilenode(Oid reltablespace, Oid relfilenode)
 
 	while (HeapTupleIsValid(ntp = systable_getnext(scandesc)))
 	{
-		bool isnull;
+		bool isnull PG_USED_FOR_ASSERTS_ONLY;
 
 		if (found)
 			elog(ERROR,