Restrictions On Trigger

Trigger :

Triggers in Oracle are blocks of code which Oracle automatically executes based on some action or event on a table.

They are mainly used for following tasks -

1. Applying complex-integrity-constraints which is not possible through complex approach of applying constraint.
2. Performing audit-trail i.e. recording changes made by other users on a table.
3. Generating Primary-key values automatically.

 Restrictions On Trigger :

Although Triggers are very powerful objects but still they suffers from few restrictions -

1. A Trigger can't issue any TCL-command (commit, rollback, savepoint). This is because trigger is never executed on its own rather it is always fired due to execution of a DML-statement (Insert, Update, Delete). Now at the server-end both, the trigger & the triggering-event are merged as a single block i.e. Server sees both of them as a single unit of transaction.Thus trigger alone can't use COMMIT or ROLLBACK. So when at the SQL prompt, the triggering statement is committed or rolled-back automatically.
2. If the trigger from its body is calling any stored procedure or function then that function or procedure also can't use commit or rollback.
3. There are restrictions on the table which a trigger can access. But to understand these restrictions, we will have to understand two terms, called Mutating Table and Constraining Table.

A "Mutating table" in Oracle is one which is currently undergoing any kind of change. For a trigger, it is simply the table on which trigger has been created.

A "Constraining table" on the other hand is one which is serving as 'parent-table' for the 'mutating table'.

Considering the above tables, the restrictions are -

• A trigger can't access any column of the mutating-table from its body. In other words, 

1. A row-level trigger cannot query or modify a mutating table. (Of course, NEW and OLD still can be accessed by the trigger) .
2. A statement-level trigger must not query or modify a mutating table if the trigger is fired as the result of a CASCADE delete. (A foreign key with a cascade delete means that if a record in the parent table is deleted, then the corresponding records in the child table with automatically be deleted. This is called a cascade delete.)

• The statements of a trigger cannot change the PRIMARY, FOREIGN, or UNIQUE KEY columns of a constraining table of the triggering statement. However, staring with Oracle 8i, there is no such restriction.

 Example :

Whenever we try to access the column of the mutating-table form trigger's body, Oracle have encountered :
ORA-04091 (table xxx is mutating. Trigger/function might not see it)

Also, please keep in mind following points :

• The trigger body cannot declare any LONG or LONG RAW variables. Also, :new and :old cannot refer to a LONG or LONG RAW column in the table for which the trigger is defined.
• Code in a trigger-body may reference and use LOB (Large Object) columns, but it may not modify the values of the columns.

Question: How many triggers can be applied to a table?

Answer: A maximum of 12 triggers can be applied to one table.

Question: Explain the uses of database trigger.

Answer: A PL/SQL program unit associated with a particular database table is called a database trigger. It is used for :

1)Audit data modifications.

2)Log events transparently.

3)Enforce complex business rules.

4)Maintain replica tables

5)Derive column values

6)Implement Complex security authorizations