Error Conditions and Exceptions in the Trigger Body If a predefined or user-defined error condition or exception is raised during the execution of a trigger body, then all effects of the trigger body, as well as the triggering statement, are rolled back unless the error is trapped by an exception handler. Compilation involves three stages: After you have completed the task that required the trigger to be disabled, re-enable the trigger, so that it fires when appropriate. It must be replaced with a new definition. Within the code of the trigger body, you can execute blocks of code depending on the kind of DML operation fired the trigger: However, pay special attention when calling remote procedures from within a local trigger. Who Is the Trigger User? To avoid this problem, you must forbid multirow updates to p that change the primary key and reuse existing primary key values. If the object is read only, then it is not necessary to define triggers to pin it. The previous example exception statement cannot run, because the trigger does not complete compilation. A loop-back database link makes a local table appear remote by defining an Oracle Net path back to the database that contains the link. System Trigger Restrictions Depending on the event, different event attribute functions are available. You may need to set up data structures for certain examples to work: An attempt is made to validate the trigger on occurrence of the event. Enabling Triggers By default, a trigger is automatically enabled when it is created; however, it can later be disabled. Check "Event Attribute Functions" before using an event attribute function, because its effects might be undefined rather than producing an error condition. For example, if a table p has three rows with the values 1 , 2 , 3 , and table f also has three rows with the values 1 , 2 , 3 , then the following statement updates p correctly but causes problems when the trigger updates f:
Enabling and Disabling Triggers A trigger can be in one of two distinct modes: Can only be defined over nested table columns in views. This way, the trigger can minimize its overhead when the column of interest is not being changed. Runtime that trigger errors always cause the DML statement to fail. For example, triggers are commonly used to: This restriction prevents a trigger from seeing an inconsistent set of data. In more realistic examples, you might test if one column value is less than another. For example, if you create a trigger that should be fired after all CREATE events, then the trigger itself does not fire after the creation, because the correct information about this trigger was not committed at the time when the trigger on CREATE events was fired. When a trigger encounters a mutating table, a runtime error occurs, the effects of the trigger body and triggering statement are rolled back, and control is returned to the user or application. You need to perform a large data load, and you want it to proceed quickly without firing triggers. For example, you define the following trigger: A loop-back database link makes a local table appear remote by defining an Oracle Net path back to the database that contains the link. Oracle Database Concepts for information about the interaction of triggers and integrity constraints Because declarative referential integrity constraints are not supported between tables on different nodes of a distributed database, the mutating table restrictions do not apply to triggers that access remote nodes. Because this is rare, this option is infrequently used. For example, assume a trigger is defined as the following: Only committed triggers are fired. The actions then insert new rows or update existing rows, as appropriate. DDL statements are not allowed in the body of a trigger. Because stored procedures are stored in a compiled form, the work-around for the previous example is as follows: This is similar to the privilege model for stored procedures. However, you cannot modify: System Trigger Restrictions Depending on the event, different event attribute functions are available. Dependencies for Triggers Compiled triggers have dependencies. Triggers and Handling Remote Exceptions A trigger that accesses a remote site cannot do remote exception handling if the network link is unavailable. Thus, if a remote site is unavailable when the trigger must compile, then Oracle Database cannot validate the statement accessing the remote database, and the compilation fails. The previous example exception statement cannot run, because the trigger does not complete compilation.
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