Some people would argue that defining a primary key to be 'NOT NULL' is redundant as well, so a statement without it should be considered the same. (I'm not sure if that's a function of the database, though, which might introduce extra complexity).

PS. back when I took Oracle's SQL Tuning class (back in 2001 or so), we were told that the caching only worked for exact string matches -- any whitespace or capitalization changes would be considered a different query to the optimizer.