Creating the trigger function

The trigger function definition looks mostly like an ordinary function definition, except that it has a return value type trigger, and it does not take any arguments:

CREATE FUNCTION mytriggerfunc() RETURNS trigger AS $$ …

Trigger functions are passed information about their calling environment through a special TriggerData structure, which in the case of PL/pgSQL is accessible through a set of local variables. The local variables, OLD and NEW, represent the row the trigger is in the before and after states of the triggering event. Additionally, there are several other local variables starting with the prefix TG_ such as TG_WHEN or TG_TABLE_NAME for general context. Once your trigger function is defined, you can bind it to a specific set of actions on a table.

Creating the trigger

The simplified syntax for creating a user-defined TRIGGER statement is given as follows:

CREATE TRIGGER name
    { BEFORE | AFTER | INSTEAD OF } { event [ OR ... ] }
    ON table_name
...