How is a trigger compiled?

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How is a trigger compiled?

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A trigger is similar to a PL/SQL anonymous block, but with the additional :NEW and :OLD capabilities. However, their compilation is different. A PL/SQL anonymous block is compiled each time it is loaded into the memory. The compilation involves the following three stages:

Syntax checking: PL/SQL syntax is checked and a parse tree is generated.

Semantic checking: Type checking and further processing on the parse tree are performed.

Code generation: The pcode is generated.

A trigger, in contrast, is fully compiled when the CREATE TRIGGER statement is executed. The pcode is stored in the data dictionary. Hence, firing the trigger no longer requires the opening of a shared cursor to run the trigger action. Instead, the trigger is executed directly.

If errors occur during the compilation of a trigger, the trigger is still created. However, if a Data Manipulation Language (DML) statement fires this trigger, the DML statement fails. The SHOW ERRORS statement can be used within SQL*Plus or Enterprise Manager to see the compilation errors that occur during the creation of a trigger. The USER_ERRORS data dictionary view can also be queried to see the compilation errors.

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