Add a WHEN clause to CREATE TRIGGER, allowing a boolean expression to be
checked to determine whether the trigger should be fired. For BEFORE triggers this is mostly a matter of spec compliance; but for AFTER triggers it can provide a noticeable performance improvement, since queuing of a deferred trigger event and re-fetching of the row(s) at end of statement can be short-circuited if the trigger does not need to be fired. Takahiro Itagaki, reviewed by KaiGai Kohei.
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