Given your comment, if you need to optimize your DB queries, and secure your server on the other hand, you probably need two people.
First about the server security: most of coders will not have skills/knowledge for truly securing a whole server (well configured firewall, file rights, uninstall of unwanted packages/apps, compilation of last versions of used packages, ssh access limitations, and many more depending on what your app do).
Concerning your DB queries, it depends on how big is your app, but it would have been better to think about optimizations at the beginning, because optimizing the queries may require code rewriting. Anyway, as you're here now, try to get a freelancer specialized in such tasks.
In fact I think you don't need a code review, but a system administrator and a database engineer. That's not the same at all.
So I would answer as @Kekito, adding odesk.com to the list of freelancers sites. Be very aware of who you hire: I had myself the best and the worst experiences using freelancers using this sites. Be sure to fully understand what you ask to them, in order to check in detail their work, given the fact you won't know them at the beginning.
In my opinion, for such things it would be better to hire people not too far from you, in order to work with them and follow carefully what they do - to control first, but above all to keep full control on your server/code once they're gone.