Mastering PostgreSQL 12 - Third Edition

Writing triggers in PL/pgSQL

Server-side code is especially popular if you want to react to certain events that are happening in the database. A trigger allows you to call a function if an INSERT, UPDATE, DELETE, or TRUNCATE clause happens on a table. The function that is called by the trigger can then modify the data that's changed in your table, or simply perform a necessary operation.

In PostgreSQL, triggers have become more powerful over the years, and they now provide a rich set of features:

test=# \h CREATE TRIGGERCommand: CREATE TRIGGERDescription: define a new triggerSyntax:CREATE [ CONSTRAINT ] TRIGGER name { BEFORE | AFTER | INSTEAD OF } { event [ OR ... ] }    ON table_name    [ FROM referenced_table_name ] [ NOT DEFERRABLE | [ DEFERRABLE ...