diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index d04dd747af..5126f14eb6 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -16946,7 +16946,11 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
    <para>
     <function>pg_database_size</function> and <function>pg_tablespace_size</>
     accept the OID or name of a database or tablespace, and return the total
-    disk space used therein.
+    disk space used therein.  To use <function>pg_database_size</function>,
+    you must have <literal>CONNECT</> permission on the specified database
+    (which is granted by default).  To use <function>pg_tablespace_size</>,
+    you must have <literal>CREATE</> permission on the specified tablespace,
+    unless it is the default tablespace for the current database.
    </para>
 
    <para>