Event triggers are created using the CREATE EVENT TRIGGER command. Before you can create an event trigger, you need a function that the trigger will execute. This function must return a special type called EVENT_TRIGGER. If you happen to define multiple event triggers, they are executed in the alphabetical order of their names.
Currently, event triggers are supported on three events, as follows:
ddl_command_start: This event occurs just before aCREATE,ALTER, orDROP DDLcommand is executedddl_command_end: This event occurs just after aCREATE,ALTER, orDROPcommand has finished executingsql_drop: This event occurs just before theddl_command_endevent for the commands that drop database objects
You can specify a WHEN clause with the CREATE EVENT TRIGGER command, so that the event is fired only for the specified commands.
The event trigger PL/pgSQL functions have access to the following new variables introduced in 9.3: