Procedural languages aren't built into the PostgreSQL server; they are offered
by loadable modules. Please refer to the documentation of the
procedural language in question for details about the syntax and how the function body
is interpreted for each language.
There are currently four procedural languages available in the
standard PostgreSQL distribution:
PL/Perl, and PL/Python. Other languages can be
defined by users. Refer to Chapter 18 for more
information. The basics of developing a new procedural language are covered in Section 9.8.