Oracle 11g Compound Trigger

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In Oracle 11g, the concept of compound trigger was introduced. A compound trigger is a single trigger on a table that enables you to specify actions for each of four timing points:

1. Before the firing statement
2. Before each row that the firing statement affects
3. After each row that the firing statement affects
4. After the firing statement

With the compound trigger, both the statement-level and row-level action can be put up in a single trigger. Plus there is an added advantage: it allows sharing of common state between all the trigger-points using variable. This is because compound trigger in oracle 11g has a declarative section where one can declare variable to be used within trigger. This common state is established at the start of triggering statement and is destroyed after completion of trigger (regardless of trigger being in error or not). If same had to be done without compound-trigger, it might have been required to share data using packages.

When to use Compound Triggers

The compound trigger is useful when you want to accumulate facts that characterize the “for each row” changes and then act on them as a body at “after statement” time. Two popular reasons to use compound trigger are:

1. To accumulate rows for bulk-insertion. We will later see an example for this.
2. To avoid the infamous ORA-04091: mutating-table error.
   

   Details of Syntax

CREATE OR REPLACE TRIGGER compound_trigger_name
FOR [INSERT|DELETE]UPDATE [OF column] ON table
COMPOUND TRIGGER
   -- Declarative Section (optional)
   -- Variables declared here have firing-statement duration.
     
     --Executed before DML statement
     BEFORE STATEMENT IS
     BEGIN
       NULL;
     END BEFORE STATEMENT;
   
     --Executed before each row change- :NEW, :OLD are available
     BEFORE EACH ROW IS
     BEGIN
       NULL;
     END BEFORE EACH ROW;
   
     --Executed aftereach row change- :NEW, :OLD are available
     AFTER EACH ROW IS
     BEGIN
       NULL;
     END AFTER EACH ROW;
   
     --Executed after DML statement
     AFTER STATEMENT IS
     BEGIN
       NULL;
     END AFTER STATEMENT;

END compound_trigger_name;

Note the ‘COMPOUND TRIGGER’ keyword above.

Some Restriction/Catches to note

1. The body of a compound trigger must be a compound trigger block.
2. A compound trigger must be a DML trigger.
3. A compound trigger must be defined on either a table or a view.
4. The declarative part cannot include PRAGMA AUTONOMOUS_TRANSACTION.
5. A compound trigger body cannot have an initialization block; therefore, it cannot have an exception section. This is not a problem, because the BEFORE STATEMENT section always executes exactly once before any other timing-point section executes.
6. An exception that occurs in one section must be handled in that section. It cannot transfer control to another section.
7. If a section includes a GOTO statement, the target of the GOTO statement must be in the same section.
8. OLD, :NEW, and :PARENT cannot appear in the declarative part, the BEFORE STATEMENT section, or the AFTER STATEMENT section.
9. Only the BEFORE EACH ROW section can change the value of :NEW.
10. If, after the compound trigger fires, the triggering statement rolls back due to a DML exception:
    • Local variables declared in the compound trigger sections are re-initialized, and any values computed thus far are lost.
    • Side effects from firing the compound trigger are not rolled back.
11. The firing order of compound triggers is not guaranteed. Their firing can be interleaved with the firing of simple triggers.
12. If compound triggers are ordered using the FOLLOWS option, and if the target of FOLLOWS does not contain the corresponding section as source code, the ordering is ignored.