Quick estimate of num of rows & table size

Hi all,

I read somewhere that the following query gives a quick estimate of the #
of rows in a table regardless of the table's size (which would matter in a
simple SELECT count(*)?):

SELECT (CASE WHEN reltuples > 0 THEN
pg_relation_size('mytable')/(8192*relpages/reltuples)
ELSE 0
END)::bigint AS estimated_row_count
FROM pg_class
WHERE oid = 'mytable'::regclass;

If relpages & reltuples are recorded accurately each time VACUUM is run,
wouldn't it be the same to just grab directly the value of reltuples like:

SELECT reltuples FROM pg_class WHERE oid='mytable'::regclass;

In the same manner, are pg_relation_size('mytable') and 8192*relpages the
same?

I run both assumptions against a freshly VACUUMed table and they seem
correct.

TIA,
Thalis K.