A trigger is a named database object that is associated with a table and that activates when a particular event occurs for the table. Activation of a trigger means execution of a set of commands attached to that trigger. For example, BEFORE trigger for INSERT statements could be used to check the values to be inserted into new rows.
Trigger functions can be written in C or in any of the available procedural languages. It is not currently possible to write a trigger function in SQL. The trigger function must be defined before the trigger itself can be created. The trigger function must be declared as a function taking no arguments and returning type trigger. In the Database Designer for PostgreSQL such functions are called procedures.
With Database Designer for PostgreSQL you can edit table triggers.