What determines if a simple query (no joins) is fully or partially optimized?

For example: I have a table RATES in MyDatabase

OPEN DATA MyDatabase
ALTER TABLE rates ADD PRIMARY KEY r_pk
SELECT * FROM rates WHERE r_pk = 5

shows the following

Using index tag R_pk to rushmore optimize table rates
Rushmore optimization level for table rates: partial

Why wouldn't this simple query be fully optimized??

Thanks to all.........Rich