
   sets of base or composite values.
  </p><p>
   Many kinds of functions can take or return certain pseudo-types
   (such as polymorphic types), but the available facilities vary.
   Consult the description of each kind of function for more details.
  </p><p>
   It's easiest to define <acronym class="acronym">SQL</acronym>
   functions, so we'll start by discussing those.
   Most of the concepts presented for <acronym class="acronym">SQL</acronym> functions
   will carry over to the other types of functio