I've written on like that a while ago:

http://webmail.postgresql.org/~petere/dbsize.html

The tarball can be rolled into contrib -- now that I think of it I don't
know why I never did that.

Never imagined this would have anything to do with that TODO item,
though.
I figured oid2name accomplished that.

Peter Eisentraut   peter_e@gmx.net

contrib/dbsize/dbsize.sql.in

@@ -0,0 +1,7 @@
+CREATE FUNCTION database_size (name) RETURNS bigint
+    AS '@MODULE_FILENAME@', 'database_size'
+    LANGUAGE C WITH (isstrict);
+
+CREATE FUNCTION relation_size (name) RETURNS bigint
+    AS '@MODULE_FILENAME@', 'relation_size'
+    LANGUAGE C WITH (isstrict);