9.27. 觸發函式

CurrentlyPostgreSQLprovides one built in trigger function,suppress_redundant_updates_trigger, which will prevent any update that does not actually change the data in the row from taking place, in contrast to the normal behavior which always performs the update regardless of whether or not the data has changed. (This normal behavior makes updates run faster, since no checking is required, and is also useful in certain cases.)

Ideally, you should normally avoid running updates that don't actually change the data in the record. Redundant updates can cost considerable unnecessary time, especially if there are lots of indexes to alter, and space in dead rows that will eventually have to be vacuumed. However, detecting such situations in client code is not always easy, or even possible, and writing expressions to detect them can be error-prone. An alternative is to usesuppress_redundant_updates_trigger, which will skip updates that don't change the data. You should use this with care, however. The trigger takes a small but non-trivial time for each record, so if most of the records affected by an update are actually changed, use of this trigger will actually make the update run slower.

Thesuppress_redundant_updates_triggerfunction can be added to a table like this:

CREATE TRIGGER z_min_update
BEFORE UPDATE ON tablename
FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();

In most cases, you would want to fire this trigger last for each row. Bearing in mind that triggers fire in name order, you would then choose a trigger name that comes after the name of any other trigger you might have on the table.

For more information about creating triggers, see CREATE TRIGGER.