Event triggers

Rules and triggers act as DML statements, which means they are triggered by something that changes the data but not the data layout or the table properties. PostgreSQL provides so-called event triggers, which are particular triggers that fire on DDL statements. The purpose of the event trigger, therefore, is to manage and react to events that will change the data structure rather than the data content. Triggers can be used in many ways to enforce specific policies across your databases.

Once fired, an event trigger receives an event and a command tag, both of which are useful for introspection and providing information about what fired the trigger. In particular, the command tag contains a description of the command (for example, CREATE or ALTER), while the event contains the category that fired the trigger – in particular, the following:

• ddl_command_start and ddl_command_end indicate, respectively, the beginning and the completion of the DDL command...