PostgreSQL triggers a function when a certain event occurs on a table, view, or foreign table. Triggers are executed when a user tries to modify the data through any of the DML events, including INSERT, UPDATE, DELETE, or TRUNCATE. The trigger synopsis is as follows:
CREATE [ CONSTRAINT ] TRIGGER name { BEFORE | AFTER | INSTEAD OF } { event [ OR ... ] } ON table_name [ FROM referenced_table_name ] [ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ] [ REFERENCING { { OLD | NEW } TABLE [ AS ] transition_relation_name } [ ... ] ] [ FOR [ EACH ] { ROW | STATEMENT } ] [ WHEN ( condition ) ] EXECUTE PROCEDURE function_name ( arguments )where event can be one of: INSERT UPDATE [ OF column_name [, ... ] ...
