Editing PostgreSQL (section)

=== Rules ===
Rules allow the "query tree" of an incoming query to be rewritten; they are an, automatically invoked, [[Macro (computer science)|macro language]] for SQL. "Query Re-Write Rules" are attached to a table/class and "Re-Write" the incoming DML (select, insert, update, and/or delete) into one or more queries that either replace the original DML statement or execute in addition to it. Query Re-Write occurs after DML statement parsing and before query planning.

The functionality rules provide was, in almost every way, later duplicated with the introduction of newer types of triggers.
The use of triggers is usually preferred over rules as it is easier to reason about trigger behavior and interactions than when equivalent rules are used.