Postgres (and other database systems) have many builtin
functions, organized by type.
  - Provides the ability to bind user-supplied code to operators, not unlike C++
      operator overloading.
  
- So in Postgres, operators and functions differ only syntactically.
  
- Kinds of functions:
      
        - Builtin.
        
- User-defined: Linked in as a shared library, (typically written in C).