You don't mention the version of Red Hat, but I assume you use a version which is not so terribly old it does not have yum.
With
yum deplist postfix
you can list all the dependencies which postfix has in the RPM package. On my Fedora system, libdb
is listed as a dependency, which is the Berkeley DB library and is sufficient for postfix.
When you install RPM's with yum, the dependencies are automatically resolved and everything which is required will be installed. Trust yum.