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22 Triggers

This chapter discusses triggers, which are procedures stored in PL/SQL or Java that run (fire) implicitly whenever a table or view is modified or when some user actions or database system actions occur.

This chapter contains the following topics:

Introduction to Triggers

You can write triggers that fire whenever one of the following operations occurs:

  1. DML statements (INSERT, UPDATE, DELETE) on a particular table or view, issued by any user

  2. DDL statements (CREATE or ALTER primarily) issued either by a particular schema/user or by any schema/user in the database

  3. Database events, such as logon/logoff, errors, or startup/shutdown, also issued either by a particular schema/user or by any schema/user in the database

Triggers are similar to stored procedures. A trigger stored in the database can include SQL and PL/SQL or Java statements to run as a unit and can invoke stored procedures. However, procedures and triggers differ in the way that they are invoked. A procedure is explicitly run by a user, application, or trigger. Triggers are implicitly fired by Oracle Database when a triggering event occurs, no matter which user is connected or which application is being used.

Figure 22-1 shows a database application with some SQL statements that implicitly fire several triggers stored in the database. Notice that the database stores triggers separately from their associated tables.

A trigger can also invoke a C procedure, which is useful for computationally intensive operations.

See Also:

This section includes the following topics:

How Triggers Are Used

Triggers supplement the standard capabilities of Oracle Database to provide a highly customized database management system. For example, a trigger can restrict DML operations against a table to those issued during regular business hours. You can also use triggers to:

  • Automatically generate derived column values

  • Prevent invalid transactions

  • Enforce complex security authorizations

  • Enforce referential integrity across nodes in a distributed database

  • Enforce complex business rules

  • Provide transparent event logging

  • Provide auditing

  • Maintain synchronous table replicates

  • Gather statistics on table access

  • Modify table data when DML statements are issued against views

  • Publish information about database events, user events, and SQL statements to subscribing applications

See Also:

Oracle Database PL/SQL Language Reference for more information about triggers

This section includes the following topics:

Some Cautionary Notes about Triggers

Although triggers are useful for customizing a database, use them only when necessary. Excessive use of triggers can result in complex interdependencies, which can be difficult to maintain in a large application. For example, when a trigger fires, a SQL statement within its trigger action potentially can fire other triggers, resulting in cascading triggers. This can produce unintended effects.

Triggers Compared with Declarative Integrity Constraints

You can use both triggers and integrity constraints to define and enforce any type of integrity rule. However, Oracle strongly recommends that you use triggers to constrain data input only in the following situations:

  • To enforce referential integrity when child and parent tables are on different nodes of a distributed database

  • To enforce complex business rules not definable using integrity constraints

  • When a required referential integrity rule cannot be enforced using the following integrity constraints:

    • NOT NULL, UNIQUE

    • PRIMARY KEY

    • FOREIGN KEY

    • CHECK

    • DELETE CASCADE

    • DELETE SET NULL

See Also:

"How Oracle Database Enforces Data Integrity" for more information about integrity constraints

Components of a Trigger

A trigger has three basic components, each explained in this section:

Figure 22-2 represents each of these trigger components and is not meant to show exact syntax. The sections that follow explain each trigger component in greater detail.

Figure 22-2 The REORDER Trigger

Description of Figure 22-2 follows
Description of "Figure 22-2 The REORDER Trigger"

The Triggering Event or Statement

A triggering event or statement is the SQL statement, database event, or user event that causes a trigger to fire. A triggering event can be one or more of the following:

  • An INSERT, UPDATE, or DELETE statement on a specific table (or view, in some cases)

  • A CREATE, ALTER, or DROP statement on any schema object

  • A database startup or instance shutdown

  • A specific error message or any error message

  • A user logon or logoff

For example, in Figure 22-2, the triggering statement is:

... UPDATE OF parts_on_hand ON inventory ...

This statement means that when the parts_on_hand column of a row in the inventory table is updated, fire the trigger. When the triggering event is an UPDATE statement, you can include a column list to identify which columns must be updated to fire the trigger. You cannot specify a column list for INSERT and DELETE statements, because they affect entire rows of information.

A triggering event can specify multiple SQL statements:

... INSERT OR UPDATE OR DELETE OF inventory ...

This part means that when an INSERT, UPDATE, or DELETE statement is issued against the inventory table, fire the trigger. When multiple types of SQL statements can fire a trigger, you can use conditional predicates to detect the type of triggering statement. In this way, you can create a single trigger that runs different code based on the type of statement that fires the trigger.

Trigger Restriction

A trigger restriction specifies a Boolean expression that must be true for the trigger to fire. The trigger action is not run if the trigger restriction evaluates to false or unknown. In the example, the trigger restriction is:

new.parts_on_hand < new.reorder_point 

Consequently, the trigger does not fire unless the number of available parts is less than a present reorder amount.

Trigger Action

A trigger action is the procedure (PL/SQL block, Java program, or C callout) that contains the SQL statements and code to be run when the following events occur:

  • A triggering statement is issued.

  • The trigger restriction evaluates to true.

Like stored procedures, a trigger action can:

  • Contain SQL, PL/SQL, or Java statements

  • Define PL/SQL language constructs such as variables, constants, cursors, exceptions

  • Define Java language constructs

  • Invoke stored procedures

If the triggers are row triggers, the statements in a trigger action have access to column values of the row being processed by the trigger. Correlation names provide access to the old and new values for each column.

Types of Triggers

This section describes the different types of triggers:

Row Triggers and Statement Triggers

When you define a trigger, you can specify the number of times the trigger action is to be run:

  • Once for every row affected by the triggering statement, such as a trigger fired by an UPDATE statement that updates many rows

  • Once for the triggering statement, no matter how many rows it affects

This section includes the following topics:

Row Triggers

A row trigger is fired each time the table is affected by the triggering statement. For example, if an UPDATE statement updates multiple rows of a table, a row trigger is fired once for each row affected by the UPDATE statement. If a triggering statement affects no rows, a row trigger is not run.

Row triggers are useful if the code in the trigger action depends on data provided by the triggering statement or rows that are affected. For example, Figure 22-2 illustrates a row trigger that uses the values of each row affected by the triggering statement.

Statement Triggers

A statement trigger is fired once on behalf of the triggering statement, regardless of the number of rows in the table that the triggering statement affects, even if no rows are affected. For example, if a DELETE statement deletes several rows from a table, a statement-level DELETE trigger is fired only once.

Statement triggers are useful if the code in the trigger action does not depend on the data provided by the triggering statement or the rows affected. For example, use a statement trigger to:

  • Make a complex security check on the current time or user

  • Generate a single audit record

BEFORE and AFTER Triggers

When defining a trigger, you can specify the trigger timingwhether the trigger action is to be run before or after the triggering statement. BEFORE and AFTER apply to both statement and row triggers.

BEFORE and AFTER triggers fired by DML statements can be defined only on tables, not on views. However, triggers on the base tables of a view are fired if an INSERT, UPDATE, or DELETE statement is issued against the view. BEFORE and AFTER triggers fired by DDL statements can be defined only on the database or a schema, not on particular tables.

See Also:

This section includes the following topics:

BEFORE Triggers

BEFORE triggers run the trigger action before the triggering statement is run. This type of trigger is commonly used in the following situations:

  • When the trigger action determines whether the triggering statement should be allowed to complete. Using a BEFORE trigger for this purpose, you can eliminate unnecessary processing of the triggering statement and its eventual rollback in cases where an exception is raised in the trigger action.

  • To derive specific column values before completing a triggering INSERT or UPDATE statement.

AFTER Triggers

AFTER triggers run the trigger action after the triggering statement is run.

Trigger Type Combinations

Using the options listed previously, you can create four types of row and statement triggers:

  • BEFORE statement trigger

    Before executing the triggering statement, the trigger action is run.

  • BEFORE row trigger

    Before modifying each row affected by the triggering statement and before checking appropriate integrity constraints, the trigger action is run, if the trigger restriction was not violated.

  • AFTER statement trigger

    After executing the triggering statement and applying any deferred integrity constraints, the trigger action is run.

  • AFTER row trigger

    After modifying each row affected by the triggering statement and possibly applying appropriate integrity constraints, the trigger action is run for the current row provided the trigger restriction was not violated. Unlike BEFORE row triggers, AFTER row triggers lock rows.

You can have multiple triggers of the same type for the same statement for any given table. For example, you can have two BEFORE statement triggers for UPDATE statements on the employees table. Multiple triggers of the same type permit modular installation of applications that have triggers on the same tables. Also, Oracle Database materialized view logs use AFTER row triggers, so you can design your own AFTER row trigger in addition to the Oracle-defined AFTER row trigger.

You can create as many triggers of the preceding different types as you need for each type of DML statement, (INSERT, UPDATE, or DELETE).

See Also:

Oracle Database PL/SQL Language Reference for more information about trigger types

Compound Triggers

A compound trigger is a single trigger on a table that enables you to specify actions for each of four timing points:

  • Before the firing statement

  • Before each row that the firing statement affects

  • After each row that the firing statement affects

  • After the firing statement

The compound trigger body supports a common PL/SQL state that the code for each timing point can access. The common state is automatically destroyed when the firing statement completes, even when the firing statement causes an error.

The effect of the compound trigger is similar to what you could achieve with a simple trigger for each of the timing points for which you needed to code action, with an ancillary package to hold the state that these simple triggers would share. The obvious advantage of the compound trigger is that the required code is managed in a single compilation unit, but the more important advantage is that the lifetime of the compound trigger's state is automatically limited to the duration of the firing statement.

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. Sometimes you are forced to use this approach (to avoid the mutating table error). Sometimes this approach gives better performance; for example, when maintaining a denormalized aggregate value in a master table in response to changes in a detail table, or when maintaining an audit table.

INSTEAD OF Triggers

INSTEAD OF triggers provide a transparent way of modifying views that cannot be modified directly through DML statements (INSERT, UPDATE, and DELETE). These triggers are invoked INSTEAD OF triggers because, unlike other types of triggers, Oracle Database fires the trigger instead of executing the triggering statement.

You can write normal INSERT, UPDATE, and DELETE statements against the view and the INSTEAD OF trigger is fired to update the underlying tables appropriately. INSTEAD OF triggers are activated for each row of the view that gets modified.

This section includes the following topics:

Modify Views

Modifying views can have ambiguous results:

  • Deleting a row in a view could either mean deleting it from the base table or updating some values so that it is no longer selected by the view.

  • Inserting a row in a view could either mean inserting a new row into the base table or updating an existing row so that it is projected by the view.

  • Updating a column in a view that involves joins might change the semantics of other columns that are not projected by the view.

Object views present additional problems. For example, a key use of object views is to represent master/detail relationships. This operation inevitably involves joins, but modifying joins is inherently ambiguous.

As a result of these ambiguities, there are many restrictions on which views are modifiable. An INSTEAD OF trigger can be used on object views as well as relational views that are not otherwise modifiable.

A view is inherently modifiable if data can be inserted, updated, or deleted without using INSTEAD OF triggers and if it conforms to the restrictions listed as follows. Even if the view is inherently modifiable, you might want to perform validations on the values being inserted, updated or deleted. INSTEAD OF triggers can also be used in this case. Here the trigger code performs the validation on the rows being modified and if valid, propagate the changes to the underlying tables.

INSTEAD OF triggers also enable you to modify object view instances on the client-side through OCI. To modify an object materialized by an object view in the client-side object cache and flush it back to the persistent store, you must specify INSTEAD OF triggers, unless the object view is inherently modifiable. However, it is not necessary to define these triggers for just pinning and reading the view object in the object cache.

Views That Are Not Modifiable

If the view query contains any of the following constructs, the view is not inherently modifiable and therefore, you cannot perform inserts, updates, or deletes on the view:

  • Set operators

  • Aggregate functions

  • GROUP BY, CONNECT BY, or START WITH clauses

  • The DISTINCT operator

  • Joins (however, some join views are updatable)

If a view contains pseudocolumns or expressions, you can only update the view with an UPDATE statement that does not refer to any of the pseudocolumns or expressions.

INSTEAD OF Triggers on Nested Tables

You cannot modify the elements of a nested table column in a view directly with the TABLE clause. However, you can do so by defining an INSTEAD OF trigger on the nested table column of the view. The triggers on the nested tables fire if a nested table element is updated, inserted, or deleted and handle the actual modifications to the underlying tables.

See Also:

Triggers on System Events and User Events

You can use triggers to publish information about database events to subscribers. Applications can subscribe to database events just as they subscribe to messages from other applications. These database events can include:

  • System events

    • Database startup and shutdown

    • Data Guard role transitions

    • Server error message events

  • User events

    • User logon and logoff

    • DDL statements (CREATE, ALTER, and DROP)

    • DML statements (INSERT, DELETE, and UPDATE)

Triggers on system events can be defined at the database level or schema level. The DBMS_AQ package is one example of using database triggers to perform certain actions. For example, a database shutdown trigger is defined at the database level:

CREATE TRIGGER register_shutdown 
  ON DATABASE 
  SHUTDOWN 
    BEGIN 
    ...
    DBMS_AQ.ENQUEUE(...); 
    ...
    END; 
    

Triggers on DDL statements or logon/logoff events can also be defined at the database level or schema level. Triggers on DML statements can be defined on a table or view. A trigger defined at the database level fires for all users, and a trigger defined at the schema or table level fires only when the triggering event involves that schema or table.

This section includes the following topics:

Event Publication

Event publication uses the publish-subscribe mechanism of Oracle Streams Advanced Queuing. A queue serves as a message repository for subjects of interest to various subscribers. Triggers use the DBMS_AQ package to enqueue a message when specific system or user events occur.

See Also:

Event Attributes

Each event allows the use of attributes within the trigger text. For example, the database startup and shutdown triggers have attributes for the instance number and the database name, and the logon and logoff triggers have attributes for the user name. You can specify a function with the same name as an attribute when you create a trigger if you want to publish that attribute when the event occurs. The attribute's value is then passed to the function or payload when the trigger fires. For triggers on DML statements, the :OLD column values pass the attribute's value to the :NEW column value.

System Events

System events that can fire triggers are related to instance startup and shutdown and error messages. Triggers created on startup and shutdown events have to be associated with the database. Triggers created on error events can be associated with the database or with a schema.

  • STARTUP triggers fire when the database is opened by an instance. Their attributes include the system event, instance number, and database name.

  • SHUTDOWN triggers fire just before the server starts shutting down an instance. You can use these triggers to make subscribing applications shut down completely when the database shuts down. For abnormal instance shutdown, these triggers cannot be fired. The attributes of SHUTDOWN triggers include the system event, instance number, and database name.

  • SERVERERROR triggers fire when a specified error occurs, or when any error occurs if no error number is specified. Their attributes include the system event and error number.

  • DB_ROLE_CHANGE triggers fire when a role transition (failover or switchover) occurs in a Data Guard configuration. The trigger notifies users when a role transition occurs, so that client connections can be processed on the new primary database and applications can continue to run.

User Events

User events that can fire triggers are related to user logon and logoff, DDL statements, and DML statements.

Triggers on LOGON and LOGOFF Events

LOGON and LOGOFF triggers can be associated with the database or with a schema. Their attributes include the system event and user name, and they can specify simple conditions on USERID and USERNAME.

  • LOGON triggers fire after a successful logon of a user.

  • LOGOFF triggers fire at the start of a user logoff.

Triggers on DDL Statements

DDL triggers can be associated with the database or with a schema. Their attributes include the system event, the type of schema object, and its name. They can specify simple conditions on the type and name of the schema object, as well as functions like USERID and USERNAME.

Triggers on DML Statements

DML triggers for event publication are associated with a table. They can be either BEFORE or AFTER triggers that fire for each row on which the specified DML operation occurs. You cannot use INSTEAD OF triggers on views to publish events related to DML statements—instead, you can publish events using BEFORE or AFTER triggers for the DML operations on a view's underlying tables that are caused by INSTEAD OF triggers.

See Also:

Trigger Execution

A trigger is either enabled or disabled.

Table 22-1 Trigger Modes

Trigger Mode Definition

Enabled

An enabled trigger runs its trigger action if a triggering statement is issued and the trigger restriction (if any) evaluates to true.

Disabled

A disabled trigger does not run its trigger action, even if a triggering statement is issued and the trigger restriction (if any) would evaluate to true.


For enabled triggers, Oracle Database automatically performs the following actions:

This section includes the following topics:

The Execution Model for Triggers and Integrity Constraint Checking

When a statement in a trigger body causes another trigger to fire, the triggers are said to be cascading. Oracle Database allows up to 32 triggers to cascade at simultaneously.

A relational database does not guarantee the order of rows processed by a SQL statement. Therefore, do not create triggers that depend on the order in which rows are processed.

See Also:

Oracle Database PL/SQL Language Reference for more information about creating triggers and the order in which they fire

Data Access for Triggers

When a trigger is fired, the tables referenced in the trigger action might be currently undergoing changes by SQL statements in other users' transactions. In all cases, the SQL statements running within triggers follow the common rules used for standalone SQL statements. In particular, if an uncommitted transaction has modified values that a trigger being fired either must read (query) or write (update), then the SQL statements in the body of the trigger being fired use the following guidelines:

  • Queries see the current read-consistent materialized view of referenced tables and any data changed within the same transaction.

  • Updates wait for existing data locks to be released before proceeding.

Storage of PL/SQL Triggers

Oracle Database stores PL/SQL triggers in compiled form, just like stored procedures. When a CREATE TRIGGER statement commits, the compiled PL/SQL code is stored in the database and the source code of the trigger is flushed from the shared pool.

See Also:

Oracle Database PL/SQL Language Reference for more information about compiling and storing triggers

Execution of Triggers

Oracle Database runs a trigger internally using the same steps used for subprogram execution. The only subtle difference is that a user has the right to fire a trigger if he or she has the privilege to run the triggering statement. Other than this difference, triggers are validated and run the same way as stored subprograms.

See Also:

Oracle Database PL/SQL Language Reference for more information about stored subprograms

Dependency Maintenance for Triggers

Like procedures, triggers depend on referenced objects. Oracle Database automatically manages the dependencies of a trigger on the schema objects referenced in its trigger action. The dependency issues for triggers are the same as those for stored procedures. Triggers are treated like stored procedures. They are inserted into the data dictionary.