For Oracle (any version > V7), your PK must have an index and
your other key, you say, is indexed so if you order by either of
these columns that are indexed, the query will not take very much
longer that your #2 statement.
You mention "approximately" so.... this'll really have to do with
how many records are in the database. If all you have is a four (4)
column table with, lets say, < 5,000 rows, querying that table
and accessing index columns (in either the "where" or "Order by"
clause, will be very fast.
For exact results and complete details as to what the query will
do, you'd need to "anaylze" the query.