pg_similarity is an extension to support similarity queries on PostgreSQL. The
implementation is tightly integrated in the RDBMS in the sense that it defines
operators so instead of the traditional operators (= and <>) you can use ~~~
and ! (any of these operators represents a similarity function).

pg_similarity has three main components:

* Functions: a set of functions that implements similarity algorithms available
  in the literature. These functions can be used as UDFs and, will be the base
  for implementing the similarity operators;
* Operators: a set of operators defined at the top of similarity functions. They
  use similarity functions to obtain the similarity threshold and, compare its
  value to a user-defined threshold to decide if it is a match or not;
* Session Variables: a set of variables that store similarity function
  parameters. Theses variables can be defined at run time.

WWW: http://pgsimilarity.projects.pgfoundry.org