| Oracle® Database Concepts 10g Release 1 (10.1) Part Number B10743-01 |
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| Oracle® Database Concepts 10g Release 1 (10.1) Part Number B10743-01 |
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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 (INSERT, UPDATE, DELETE) on a particular table or view, issued by any user
DDL statements (CREATE or 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.
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 (INSERT, UPDATE, or DELETE)
DDL statements
System events such as startup, shutdown, and error messages
User events such as logon and logoff
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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. |
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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
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
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.
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
A trigger has three basic parts:
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.
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 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 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.
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.
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
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:
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
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. 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.
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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.
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 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).
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See Also: Oracle Database Application Developer's Guide - Fundamentals for examples of trigger applications |
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 called INSTEAD OF triggers because, unlike other types of triggers, Oracle 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.
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.
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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:
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.
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.
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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
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.
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.
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.
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. DDL triggers include the following types of triggers:
BEFORE CREATE and AFTER CREATE triggers fire when a schema object is created in the database or schema.
BEFORE ALTER and AFTER ALTER triggers fire when a schema object is altered in the database or schema.
BEFORE DROP and AFTER 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 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.
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 UID, USER, USERENV, and SYSDATE), pseudocolumns, and columns. The columns can be prefixed by :OLD and :NEW for old and new values. Triggers on DML statements include the following triggers:
BEFORE INSERT and AFTER INSERT triggers fire for each row inserted into the table.
BEFORE UPDATE and AFTER UPDATE triggers fire for each row updated in the table.
BEFORE DELETE and AFTER DELETE triggers fire for each row deleted from the table.
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A trigger is in either of two distinct modes:
For enabled triggers, Oracle automatically performs the following actions:
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 performs integrity constraint checking at a set point in time with respect to the different types of triggers and guarantees that triggers cannot compromise integrity constraints.
Oracle provides read-consistent views for queries and constraints.
Oracle manages the dependencies among triggers and schema objects referenced in the code of the trigger action
Oracle uses two-phase commit if a trigger updates remote tables in a distributed database.
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 single SQL statement can potentially fire up to four types of triggers:
BEFORE row triggers
BEFORE statement triggers
AFTER row triggers
AFTER statement triggers
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:
Run all BEFORE statement triggers that apply to the statement.
Loop for each row affected by the SQL statement.
Run all BEFORE 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.)
Run all AFTER row triggers that apply to the statement.
Complete deferred integrity constraint checking.
Run all AFTER statement triggers that apply to the statement.
The definition of the execution model is recursive. For example, a given SQL statement can cause a BEFORE row trigger to be fired and an integrity constraint to be checked. That BEFORE row trigger, in turn, might perform an update that causes an integrity constraint to be checked and an AFTER statement trigger to be fired. The AFTER 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.
BEFORE row triggers fired.
AFTER statement triggers fired by UPDATE in BEFORE row trigger.
i. Statements of AFTER statement triggers run.
ii. Integrity constraint checked on tables changed by AFTER statement triggers.
Statements of BEFORE row triggers run.
Integrity constraint checked on tables changed by BEFORE row triggers.
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 DELETE CASCADE and DELETE SET 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 BEFORE row trigger, and the fired AFTER statement trigger are rolled back.
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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, allBEFORE row triggers for a single UPDATE statement may not always fire in the same order. Design your applications so they do not rely on the firing order of multiple triggers of the same type. |
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 CREATE 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.
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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.
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.
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