PostgreSQL provides four kinds of functions:
query language functions (functions written in SQL)
procedural language functions (functions written in, for example, PL/Tcl or PL/pgSQL)
C language functions
Every kind of function can take a base type, a composite type, or some combination as arguments (parameters). In addition, every kind of function can return a base type or a composite type. It's easiest to define SQL functions, so we'll start with those. Examples in this section can also be found in funcs.sql and funcs.c in the tutorial directory.
Throughout this chapter, it can be useful to look at the reference page of the CREATE FUNCTION command to understand the examples better.