This chapter provides general information about writing trigger functions. Trigger functions can be written in most of the available procedural languages, including PL/pgSQL (Chapter 39), PL/Tcl (Chapter 40), PL/Perl (Chapter 41), and PL/Python (Chapter 42). After reading this chapter, you should consult the chapter for your favorite procedural language to find out the language-specific details of writing a trigger in it.
It is also possible to write a trigger function in C, although most people find it easier to use one of the procedural languages. It is not currently possible to write a trigger function in the plain SQL function language.
Read the upstream documentation. For corrections, first check the current version.