|Oracle® Database Concepts
10g Release 1 (10.1)
Part Number B10743-01
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:
You can write triggers that fire whenever one of the following operations occurs:
DML statements (
DELETE) on a particular table or view, issued by any user
DDL statements (
ALTER primarily) issued either by a particular schema/user or by any schema/user in the database
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 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.
Figure 22-1 Triggers
A trigger can also call out to a C procedure, which is useful for computationally intensive operations.
The events that fire a trigger include the following:
DML statements that modify data in a table (
System events such as startup, shutdown, and error messages
User events such as logon and logoff
Note:Oracle Forms can define, store, and run triggers of a different sort. However, do not confuse Oracle Forms triggers with the triggers discussed in this chapter.
Triggers supplement the standard capabilities of Oracle 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
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 Application Developer's Guide - Fundamentals for examples of trigger uses
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. Figure 22-2 illustrates cascading triggers.
Figure 22-2 Cascading Triggers
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:
See Also:"How Oracle Enforces Data Integrity " for more information about integrity constraints
A triggering event or statement
A trigger restriction
A trigger action
Figure 22-3 represents each of these parts of a trigger and is not meant to show exact syntax. The sections that follow explain each part of a trigger in greater detail.
Figure 22-3 The REORDER Trigger
A database startup or instance shutdown
A specific error message or any error message
A user logon or logoff
For example, in Figure 22-3, 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
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
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.
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
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.
A triggering statement is issued.
The trigger restriction evaluates to
Contain SQL, PL/SQL, or Java statements
Define PL/SQL language constructs such as variables, constants, cursors, exceptions
Define Java language constructs
Call 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.
This section describes the different types of triggers:
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
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 22-3 illustrates a row trigger that uses the values of each row affected by the triggering statement.
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
When defining a trigger, you can specify the trigger timing—whether the trigger action is to be run before or after the triggering statement.
AFTER apply to both statement and row triggers.
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
DELETE statement is issued against the view.
AFTER triggers fired by DDL statements can be defined only on the database or a schema, not on particular tables.
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.
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
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
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 materialized view logs use
row triggers, so you can design your own
row trigger in addition to the Oracle-defined
You can create as many triggers of the preceding different types as you need for each type of DML statement, (
See Also:Oracle Database Application Developer's Guide - Fundamentals for examples of trigger applications
OF triggers provide a transparent way of modifying views that cannot be modified directly through DML statements (
DELETE). These triggers are called
OF triggers because, unlike other types of triggers, Oracle fires the trigger instead of executing the triggering statement.
You can write normal
DELETE statements against the view and the
OF trigger is fired to update the underlying tables appropriately.
OF triggers are activated for each row of the view that gets modified.
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
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
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.
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.
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
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.
If the view query contains any of the following constructs, the view is not inherently modifiable and you therefore cannot perform inserts, updates, or deletes on the view:
Joins (however, some join views are updatable)
See Also:"Updatable Join Views "
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
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.
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:
Database startup and shutdown
Server error message events
User logon and logoff
DDL statements (
DML statements (
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.
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.
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 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.
User events that can fire triggers are related to user logon and logoff, DDL statements, and DML statements.
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
LOGON triggers fire after a successful logon of a user.
LOGOFF triggers fire at the start of a user logoff.
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
USERNAME. DDL triggers include the following types of triggers:
CREATE triggers fire when a schema object is created in the database or schema.
ALTER triggers fire when a schema object is altered in the database or schema.
DROP triggers fire when a schema object is dropped from the database or schema.
DML triggers for event publication are associated with a table. They can be either
AFTER triggers that fire for each row on which the specified DML operation occurs. You cannot use
OF triggers on views to publish events related to DML statements—instead, you can publish events using
AFTER triggers for the DML operations on a view's underlying tables that are caused by
The attributes of DML triggers for event publication include the system event and the columns defined by the user in the
SELECT list. They can specify simple conditions on the type and name of the schema object, as well as functions (such as
SYSDATE), pseudocolumns, and columns. The columns can be prefixed by
:NEW for old and new values. Triggers on DML statements include the following triggers:
INSERT triggers fire for each row inserted into the table.
UPDATE triggers fire for each row updated in the table.
DELETE triggers fire for each row deleted from the table.
A trigger is in either of two distinct modes:
|Enabled||An enabled trigger runs its trigger action if a triggering statement is issued and the trigger restriction (if any) evaluates to |
|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 |
Oracle runs triggers of each type in a planned firing sequence when more than one trigger is fired by a single SQL statement. First, statement level triggers are fired, and then row level triggers are fired.
Oracle fires multiple triggers in an unspecified, random order, if more than one trigger of the same type exists for a given statement; that is, triggers of the same type for the same statement are not guaranteed to fire in any specific order.
A triggering statement or a statement within a trigger can cause one or more integrity constraints to be checked. Also, triggers can contain statements that cause other triggers to fire (cascading triggers).
Oracle uses the following execution model to maintain the proper firing sequence of multiple triggers and constraint checking:
statement triggers that apply to the statement.
Loop for each row affected by the SQL statement.
row triggers that apply to the statement.
Lock and change row, and perform integrity constraint checking. (The lock is not released until the transaction is committed.)
row triggers that apply to the statement.
Complete deferred integrity constraint checking.
statement triggers that apply to the statement.
The definition of the execution model is recursive. For example, a given SQL statement can cause a
row trigger to be fired and an integrity constraint to be checked. That
row trigger, in turn, might perform an update that causes an integrity constraint to be checked and an
statement trigger to be fired. The
statement trigger causes an integrity constraint to be checked. In this case, the execution model runs the steps recursively, as follows:
Original SQL statement issued.
row triggers fired.
statement triggers fired by
i. Statements of
statement triggers run.
ii. Integrity constraint checked on tables changed by
row triggers run.
Integrity constraint checked on tables changed by
SQL statement run.
Integrity constraint from SQL statement checked.
There are two exceptions to this recursion:
When a triggering statement modifies one table in a referential constraint (either the primary key or foreign key table), and a triggered statement modifies the other, only the triggering statement will check the integrity constraint. This allows row triggers to enhance referential integrity.
Statement triggers fired due to
NULL are fired before and after the user
DELETE statement, not before and after the individual enforcement statements. This prevents those statement triggers from encountering mutating errors.
An important property of the execution model is that all actions and checks done as a result of a SQL statement must succeed. If an exception is raised within a trigger, and the exception is not explicitly handled, all actions performed as a result of the original SQL statement, including the actions performed by fired triggers, are rolled back. Thus, integrity constraints cannot be compromised by triggers. The execution model takes into account integrity constraints and disallows triggers that violate declarative integrity constraints.
For example, in the previously outlined scenario, suppose that the integrity constraint is violated. As a result of this violation, all changes made by the SQL statement, the fired
row trigger, and the fired
statement trigger are rolled back.
Note:Although triggers of different types are fired in a specific order, triggers of the same type for the same statement are not guaranteed to fire in any specific order. For example, all
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 run 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 needs to 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.
Oracle stores PL/SQL triggers in compiled form, just like stored procedures. When a
TRIGGER statement commits, the compiled PL/SQL code, called P code (for pseudocode), is stored in the database and the source code of the trigger is flushed from the shared pool.
See Also:PL/SQL User's Guide and Reference for more information about compiling and storing PL/SQL code
Oracle runs a trigger internally using the same steps used for procedure 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, triggers are validated and run the same way as stored procedures.
See Also:PL/SQL User's Guide and Reference for more information about stored procedures
Like procedures, triggers depend on referenced objects. Oracle 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.