While it is relatively easy to perform trigger actions conditionally inside the PL/pgSQL trigger function, it is often more efficient to skip invoking the trigger altogether. The performance effects of firing a trigger is not generally noticed when only a few events are fired. However, if you are bulk loading data or updating large portions of your table, the cumulative effects can certainly be felt. To avoid the overhead, its best to only call the trigger function when it is actually needed.
There are two ways to narrow down when a trigger is called in the CREATE TRIGGER command itself.
So once more use the same syntax, but this time with all options:
CREATE TRIGGER name
{ BEFORE | AFTER | INSTEAD OF } { event [ OR event ... ] }
[ OF column_name [ OR column_name ... ] ] ON table_name
[ FOR [ EACH ] { ROW | STATEMENT } ]
[ WHEN ( condition ) ]
EXECUTE PROCEDURE function_name ( arguments )