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Oracle provides a more complex trigger facility than either the Informix or Transact-SQL facility described in the preceding sections. It uses a CREATE TRIGGER statement to specify triggered actions. As in the Informix facility, a trigger can be specified to fire at specific times during specific update operations: Statement-level trigger A statement-level trigger fires once for each data modification statement. It can be specified to fire either before the statement is executed or after the statement has completed its action. Row-level trigger A row-level trigger fires once for each row being modified by a statement. In Oracle s structure, this type of trigger may also fire either before the row is modified or after it is modified. Instead-of trigger An instead-of trigger takes the place of an attempted data modification statement. It provides a way to detect an attempted UPDATE, INSERT, or DELETE operation by a user or procedure, and to substitute other processing instead. You can specify that a trigger should be executed instead of a statement, or that it should be executed instead of each attempted modification of a row. System event trigger A trigger that fires when a particular system event takes place, such as a user connecting to the database, or entry of a database shutdown command.
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The following code is a PL/SQL trigger definition that implements the same processing as in the complex Informix example from the previous section. It has been split into three separate Oracle CREATE TRIGGER statements; one each for the BEFORE and AFTER statement-level triggers and one trigger that is executed for each update row.
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create trigger bef_upd_ord before update on orders begin /* Calculate order total before changes */ old_total = add_orders(); end; create trigger aft_upd_ord after update on orders begin /* Calculate order total after changes */ new_total = add_orders(); end; create trigger dur_upd_ord before update of amount on orders referencing old as pre new as post /* Capture order increases and decreases */ for each row when (:post.amount != :pre.amount) begin if post.amount != :pre.amount) then if (:post.amount < :pre.amount) then /* Write decrease data into table */ insert into ord_less values (:pre.cust, :pre.order_date, :pre.amount, :post.amount); elsif (:post.amount > :pre.amount) then /* Write increase data into table */ insert into ord_more values (:pre.cust, :pre.order_date, :pre.amount, :post.amount); end if; end if; end;
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These trigger structures and their options provide 14 different valid Oracle trigger types (12 resulting from a choice of INSERT/DELETE/UPDATE triggers for BEFORE or AFTER processing at the row or statement level (3 2 2), and two more from instead-of triggers at
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the statement or row level). In practice, relational databases built using Oracle don t tend to use instead-of triggers; they were introduced in Oracle8 to support some of its newer objectoriented features.
Other Trigger Considerations
Triggers pose some of the same issues for DBMS processing that UPDATE and DELETE rules present. For example, triggers can cause a cascaded series of actions. Suppose a user s attempt to update a table causes a trigger to fire, and within the body of that trigger is an UPDATE statement for another table. A trigger on that table causes the UPDATE of still another table, and so on. The situation is even worse if one of the fired triggers attempts to update the original target table that caused the firing of the trigger sequence in the first place! In this case, an infinite loop of fired triggers could result. Various DBMS systems deal with this issue in different ways. Some impose restrictions on the actions that can be taken during execution of a trigger. Others provide built-in functions that allow a trigger s body to detect the level of nesting at which the trigger is operating. Some provide a system setting that controls whether cascaded trigger processing is allowed. Finally, some provide a limit on the number of levels of nested triggers that can fire. One additional issue associated with triggers is the overhead that can result during very heavy database usage, such as when bulk data is being loaded into a database. Some DBMS brands provide the ability to selectively enable and disable trigger processing to handle this situation. Oracle, for example, provides this form of the ALTER TRIGGER statement:
ALTER TRIGGER BEF_UPD_ORD DISABLE;
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