Oracle® Database Application Developer's Guide - Fundamentals 10g Release 2 (10.2) Part Number B14251-01 |
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Triggers are procedures that are stored in the database and are implicitly run, or fired, when something happens.
Traditionally, triggers supported the execution of a PL/SQL block when an INSERT
, UPDATE
, or DELETE
occurred on a table or view. Triggers support system and other data events on DATABASE
and SCHEMA
. Oracle Database also supports the execution of PL/SQL or Java procedures.
This chapter discusses DML triggers, INSTEAD
OF
triggers, and system triggers (triggers on DATABASE
and SCHEMA
). Topics include:
Use the following guidelines when designing your triggers:
Use triggers to guarantee that when a specific operation is performed, related actions are performed.
Do not define triggers that duplicate features already built into Oracle Database. For example, do not define triggers to reject bad data if you can do the same checking through declarative integrity constraints.
Limit the size of triggers. If the logic for your trigger requires much more than 60 lines of PL/SQL code, it is better to include most of the code in a stored procedure and call the procedure from the trigger.
Use triggers only for centralized, global operations that should be fired for the triggering statement, regardless of which user or database application issues the statement.
Do not create recursive triggers. For example, creating an AFTER
UPDATE
statement trigger on the Emp_tab
table that itself issues an UPDATE
statement on Emp_tab
, causes the trigger to fire recursively until it has run out of memory.
Use triggers on DATABASE
judiciously. They are executed for every user every time the event occurs on which the trigger is created.
Triggers are created using the CREATE
TRIGGER
statement. This statement can be used with any interactive tool, such as SQL*Plus or Enterprise Manager. When using an interactive tool, a single slash (/) on the last line is necessary to activate the CREATE
TRIGGER
statement.
The following statement creates a trigger for the Emp_tab
table.
CREATE OR REPLACE TRIGGER Print_salary_changes BEFORE DELETE OR INSERT OR UPDATE ON Emp_tab FOR EACH ROW WHEN (new.Empno > 0) DECLARE sal_diff number; BEGIN sal_diff := :new.sal - :old.sal; dbms_output.put('Old salary: ' || :old.sal); dbms_output.put(' New salary: ' || :new.sal); dbms_output.put_line(' Difference ' || sal_diff); END; /
The trigger is fired when DML operations (INSERT
, UPDATE
, and DELETE
statements) are performed on the table. You can choose what combination of operations should fire the trigger.
Because the trigger uses the BEFORE
keyword, it can access the new values before they go into the table, and can change the values if there is an easily-corrected error by assigning to :NEW.
column_name
. You might use the AFTER
keyword if you want the trigger to query or change the same table, because triggers can only do that after the initial changes are applied and the table is back in a consistent state.
Because the trigger uses the FOR EACH ROW
clause, it might be executed multiple times, such as when updating or deleting multiple rows. You might omit this clause if you just want to record the fact that the operation occurred, but not examine the data for each row.
Once the trigger is created, entering the following SQL statement:
UPDATE Emp_tab SET sal = sal + 500.00 WHERE deptno = 10;
fires the trigger once for each row that is updated, in each case printing the new salary, old salary, and the difference.
The CREATE
(or CREATE
OR
REPLACE
) statement fails if any errors exist in the PL/SQL block.
Note:
The size of the trigger cannot be more than 32K.The following sections use this example to illustrate the way that parts of a trigger are specified.
See Also:
"Examples of Trigger Applications" for more realistic examples ofCREATE
TRIGGER
statementsA trigger is either a stored PL/SQL block or a PL/SQL, C, or Java procedure associated with a table, view, schema, or the database itself. Oracle Database automatically executes a trigger when a specified event takes place, which may be in the form of a system event or a DML statement being issued against the table.
Triggers can be:
DML triggers on tables.
INSTEAD
OF
triggers on views.
System triggers on DATABASE
or SCHEMA
: With DATABASE
, triggers fire for each event for all users; with SCHEMA
, triggers fire for each event for that specific user.
See Also:
Oracle Database SQL Reference for information on trigger creation syntaxYou can create triggers to be fired on any of the following:
DML statements (DELETE
, INSERT
, UPDATE
)
DDL statements (CREATE
, ALTER
, DROP
)
Database operations (SERVERERROR
, LOGON
, LOGOFF
, STARTUP
, SHUTDOWN
)
You can get certain event-specific attributes when the trigger is fired.
Creating a trigger on DATABASE
implies that the triggering event is outside the scope of a user (for example, database STARTUP
and SHUTDOWN
), and it applies to all users (for example, a trigger created on LOGON
event by the DBA).
Creating a trigger on SCHEMA
implies that the trigger is created in the current user's schema and is fired only for that user.
For each trigger, publication can be specified on DML and system events.
Trigger names must be unique with respect to other triggers in the same schema. Trigger names do not need to be unique with respect to other schema objects, such as tables, views, and procedures. For example, a table and a trigger can have the same name (however, to avoid confusion, this is not recommended).
A trigger is fired based on a triggering statement, which specifies:
The SQL statement or the system event, database event, or DDL event that fires the trigger body. The options include DELETE
, INSERT
, and UPDATE
. One, two, or all three of these options can be included in the triggering statement specification.
The table, view, DATABASE
, or SCHEMA
associated with the trigger.
Note:
Exactly one table or view can be specified in the triggering statement. If theINSTEAD
OF
option is used, then the triggering statement may only specify a view; conversely, if a view is specified in the triggering statement, then only the INSTEAD
OF
option may be used.For example, the PRINT_SALARY_CHANGES
trigger fires after any DELETE
, INSERT
, or UPDATE
on the Emp_tab
table. Any of the following statements trigger the PRINT_SALARY_CHANGES
trigger given in the previous example:
DELETE FROM Emp_tab; INSERT INTO Emp_tab VALUES ( ... ); INSERT INTO Emp_tab SELECT ... FROM ... ; UPDATE Emp_tab SET ... ;
INSERT
triggers fire during SQL*Loader conventional loads. (For direct loads, triggers are disabled before the load.)
The IGNORE
parameter of the IMP
command determines whether triggers fire during import operations:
If IGNORE
=N
(default) and the table already exists, then import does not change the table and no existing triggers fire.
If the table does not exist, then import creates and loads it before any triggers are defined, so again no triggers fire.
If IGNORE=Y
, then import loads rows into existing tables. Any existing triggers fire, and indexes are updated to account for the imported data.
An UPDATE
statement might include a list of columns. If a triggering statement includes a column list, the trigger is fired only when one of the specified columns is updated. If a triggering statement omits a column list, the trigger is fired when any column of the associated table is updated. A column list cannot be specified for INSERT
or DELETE
triggering statements.
The previous example of the PRINT_SALARY_CHANGES
trigger could include a column list in the triggering statement. For example:
... BEFORE DELETE OR INSERT OR UPDATE OF ename ON Emp_tab ...
Notes:
You cannot specify a column list for UPDATE
with INSTEAD
OF
triggers.
If the column specified in the UPDATE
OF
clause is an object column, then the trigger is also fired if any of the attributes of the object are modified.
You cannot specify UPDATE
OF
clauses on collection columns.
The BEFORE
or AFTER
option in the CREATE
TRIGGER
statement specifies exactly when to fire the trigger body in relation to the triggering statement that is being run. In a CREATE
TRIGGER
statement, the BEFORE
or AFTER
option is specified just before the triggering statement. For example, the PRINT_SALARY_CHANGES
trigger in the previous example is a BEFORE
trigger.
In general, you use BEFORE
or AFTER
triggers to achieve the following results:
Use BEFORE
row triggers to modify the row before the row data is written to disk.
Use AFTER
row triggers to obtain, and perform operations, using the row ID.
Note:
BEFORE
row triggers are slightly more efficient than AFTER
row triggers. With AFTER
row triggers, affected data blocks must be read (logical read, not physical read) once for the trigger and then again for the triggering statement. Alternatively, with BEFORE
row triggers, the data blocks must be read only once for both the triggering statement and the trigger.BEFORE Triggers Fired Multiple Times
If an UPDATE
or DELETE
statement detects a conflict with a concurrent UPDATE
, then Oracle Database performs a transparent ROLLBACK
to SAVEPOINT
and restarts the update. This can occur many times before the statement completes successfully. Each time the statement is restarted, the BEFORE
statement trigger is fired again. The rollback to savepoint does not undo changes to any package variables referenced in the trigger. Your package should include a counter variable to detect this situation.
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. For example, do not assign a value to a global package variable in a row trigger if the current value of the global variable is dependent on the row being processed by the row trigger. Also, if global package variables are updated within a trigger, then it is best to initialize those variables in a BEFORE
statement trigger.
When a statement in a trigger body causes another trigger to be fired, the triggers are said to be cascading. Oracle Database allows up to 32 triggers to cascade at any one time. However, you can effectively limit the number of trigger cascades using the initialization parameter OPEN_CURSORS
, because a cursor must be opened for every execution of a trigger.
Trigger Evaluation Order
Although any trigger can run a sequence of operations either in-line or by calling procedures, using multiple triggers of the same type enhances database administration by permitting the modular installation of applications that have triggers on the same tables.
Oracle Database executes all triggers of the same type before executing triggers of a different type. If you have multiple triggers of the same type on a single table, then Oracle Database chooses an arbitrary order to execute these triggers.
See Also:
Oracle Database Concepts for more information on the firing order of triggersEach subsequent trigger sees the changes made by the previously fired triggers. Each trigger can see the old and new values. The old values are the original values, and the new values are the current values, as set by the most recently fired UPDATE
or INSERT
trigger.
To ensure that multiple triggered actions occur in a specific order, you must consolidate these actions into a single trigger (for example, by having the trigger call a series of procedures).
An updatable view is one that lets you perform DML on the underlying table. Some views are inherently updatable, but others are not because they were created with one or more of the constructs listed in"Views that Require INSTEAD OF Triggers".
Any view that contains one of those constructs can be made updatable by using an INSTEAD OF
trigger. INSTEAD
OF
triggers provide a transparent way of modifying views that cannot be modified directly through UPDATE
, INSERT
, and DELETE
statements. These triggers are called INSTEAD
OF
triggers because, unlike other types of triggers, Oracle Database fires the trigger instead of executing the triggering statement. The trigger must determine what operation was intended and perform UPDATE
, INSERT
, or DELETE
operations directly on the underlying tables.
With an INSTEAD OF
trigger, you can write normal UPDATE
, INSERT
, and DELETE
statements against the view, and the INSTEAD
OF
trigger works invisibly in the background to make the right actions take place.
INSTEAD
OF
triggers can only be activated for each row.
Note:
The INSTEAD
OF
option can only be used for triggers created over views.
The BEFORE
and AFTER
options cannot be used for triggers created over views.
The CHECK
option for views is not enforced when inserts or updates to the view are done using INSTEAD
OF
triggers. The INSTEAD
OF
trigger body must enforce the check.
A view cannot be modified by UPDATE
, INSERT
, or DELETE
statements if the view query contains any of the following constructs:
A set operator
A DISTINCT
operator
An aggregate or analytic function
A GROUP
BY
, ORDER BY
, MODEL
, CONNECT
BY
, or START
WITH
clause
A collection expression in a SELECT
list
A subquery in a SELECT
list
A subquery designated WITH READ ONLY
Joins, with some exceptions, as documented in Oracle Database Administrator's Guide
If a view contains pseudocolumns or expressions, then you can only update the view with an UPDATE
statement that does not refer to any of the pseudocolumns or expressions.
Note:
You may need to set up the following data structures for this example to work:CREATE TABLE Project_tab ( Prj_level NUMBER, Projno NUMBER, Resp_dept NUMBER); CREATE TABLE Emp_tab ( Empno NUMBER NOT NULL, Ename VARCHAR2(10), Job VARCHAR2(9), Mgr NUMBER(4), Hiredate DATE, Sal NUMBER(7,2), Comm NUMBER(7,2), Deptno NUMBER(2) NOT NULL); CREATE TABLE Dept_tab ( Deptno NUMBER(2) NOT NULL, Dname VARCHAR2(14), Loc VARCHAR2(13), Mgr_no NUMBER, Dept_type NUMBER);
The following example shows an INSTEAD
OF
trigger for inserting rows into the MANAGER_INFO
view.
CREATE OR REPLACE VIEW manager_info AS SELECT e.ename, e.empno, d.dept_type, d.deptno, p.prj_level, p.projno FROM Emp_tab e, Dept_tab d, Project_tab p WHERE e.empno = d.mgr_no AND d.deptno = p.resp_dept; CREATE OR REPLACE TRIGGER manager_info_insert INSTEAD OF INSERT ON manager_info REFERENCING NEW AS n -- new manager information FOR EACH ROW DECLARE rowcnt number; BEGIN SELECT COUNT(*) INTO rowcnt FROM Emp_tab WHERE empno = :n.empno; IF rowcnt = 0 THEN INSERT INTO Emp_tab (empno,ename) VALUES (:n.empno, :n.ename); ELSE UPDATE Emp_tab SET Emp_tab.ename = :n.ename WHERE Emp_tab.empno = :n.empno; END IF; SELECT COUNT(*) INTO rowcnt FROM Dept_tab WHERE deptno = :n.deptno; IF rowcnt = 0 THEN INSERT INTO Dept_tab (deptno, dept_type) VALUES(:n.deptno, :n.dept_type); ELSE UPDATE Dept_tab SET Dept_tab.dept_type = :n.dept_type WHERE Dept_tab.deptno = :n.deptno; END IF; SELECT COUNT(*) INTO rowcnt FROM Project_tab WHERE Project_tab.projno = :n.projno; IF rowcnt = 0 THEN INSERT INTO Project_tab (projno, prj_level) VALUES(:n.projno, :n.prj_level); ELSE UPDATE Project_tab SET Project_tab.prj_level = :n.prj_level WHERE Project_tab.projno = :n.projno; END IF; END;
The actions shown for rows being inserted into the MANAGER_INFO
view first test to see if appropriate rows already exist in the base tables from which MANAGER_INFO
is derived. The actions then insert new rows or update existing rows, as appropriate. Similar triggers can specify appropriate actions for UPDATE
and DELETE
.
INSTEAD
OF
triggers provide the means to modify object view instances on the client-side through OCI calls.
See Also:
Oracle Call Interface Programmer's GuideTo 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 modifiable. If the object is read only, then it is not necessary to define triggers to pin it.
INSTEAD
OF
triggers can also be created over nested table view columns. These triggers provide a way of updating elements of the nested table. They fire for each nested table element being modified. The row correlation variables inside the trigger correspond to the nested table element. This type of trigger also provides an additional correlation name for accessing the parent row that contains the nested table being modified.
Note:
These triggers:Can only be defined over nested table columns in views.
Fire only when the nested table elements are modified using the THE
() or TABLE
() clauses. They do not fire when a DML statement is performed on the view.
For example, consider a department view that contains a nested table of employees.
CREATE OR REPLACE VIEW Dept_view AS SELECT d.Deptno, d.Dept_type, d.Dept_name, CAST (MULTISET ( SELECT e.Empno, e.Empname, e.Salary) FROM Emp_tab e WHERE e.Deptno = d.Deptno) AS Amp_list_ Emplist FROM Dept_tab d;
The CAST
(MULTISET
..) operator creates a multi-set of employees for each department. If you want to modify the emplist
column, which is the nested table of employees, then you can define an INSTEAD
OF
trigger over the column to handle the operation.
The following example shows how an insert trigger might be written:
CREATE OR REPLACE TRIGGER Dept_emplist_tr
INSTEAD OF INSERT ON NESTED TABLE Emplist OF Dept_view
REFERENCING NEW AS Employee
PARENT AS Department
FOR EACH ROW
BEGIN
-- The insert on the nested table is translated to an insert on the base table:
INSERT INTO Emp_tab VALUES (
:Employee.Empno, :Employee.Empname,:Employee.Salary, :Department.Deptno);
END;
Any INSERT
into the nested table fires the trigger, and the Emp_tab
table is filled with the correct values. For example:
INSERT INTO TABLE (SELECT d.Emplist FROM Dept_view d WHERE Deptno = 10) VALUES (1001, 'John Glenn', 10000);
The :department
.deptno
correlation variable in this example would have a value of 10.
The FOR
EACH
ROW
option determines whether the trigger is a row trigger or a statement trigger. If you specify FOR
EACH
ROW
, then the trigger fires once for each row of the table that is affected by the triggering statement. The absence of the FOR
EACH
ROW
option indicates that the trigger fires only once for each applicable statement, but not separately for each row affected by the statement.
For example, you define the following trigger:
Note:
You may need to set up the following data structures for certain examples to work:CREATE TABLE Emp_log ( Emp_id NUMBER, Log_date DATE, New_salary NUMBER, Action VARCHAR2(20));
CREATE OR REPLACE TRIGGER Log_salary_increase AFTER UPDATE ON Emp_tab FOR EACH ROW WHEN (new.Sal > 1000) BEGIN INSERT INTO Emp_log (Emp_id, Log_date, New_salary, Action) VALUES (:new.Empno, SYSDATE, :new.SAL, 'NEW SAL'); END;
Then, you enter the following SQL statement:
UPDATE Emp_tab SET Sal = Sal + 1000.0 WHERE Deptno = 20;
If there are five employees in department 20, then the trigger fires five times when this statement is entered, because five rows are affected.
The following trigger fires only once for each UPDATE
of the Emp_tab
table:
CREATE OR REPLACE TRIGGER Log_emp_update AFTER UPDATE ON Emp_tab BEGIN INSERT INTO Emp_log (Log_date, Action) VALUES (SYSDATE, 'Emp_tab COMMISSIONS CHANGED'); END;
See Also:
Oracle Database Concepts for the order of trigger firingThe statement level triggers are useful for performing validation checks for the entire statement.
Optionally, a trigger restriction can be included in the definition of a row trigger by specifying a Boolean SQL expression in a WHEN
clause.
Note:
AWHEN
clause cannot be included in the definition of a statement trigger.If included, then the expression in the WHEN
clause is evaluated for each row that the trigger affects.
If the expression evaluates to TRUE
for a row, then the trigger body is fired on behalf of that row. However, if the expression evaluates to FALSE
or NOT
TRUE
for a row (unknown, as with nulls), then the trigger body is not fired for that row. The evaluation of the WHEN
clause does not have an effect on the execution of the triggering SQL statement (in other words, the triggering statement is not rolled back if the expression in a WHEN
clause evaluates to FALSE
).
For example, in the PRINT_SALARY_CHANGES
trigger, the trigger body is not run if the new value of Empno
is zero, NULL
, or negative. In more realistic examples, you might test if one column value is less than another.
The expression in a WHEN
clause of a row trigger can include correlation names, which are explained later. The expression in a WHEN
clause must be a SQL expression, and it cannot include a subquery. You cannot use a PL/SQL expression (including user-defined functions) in the WHEN
clause.
Note:
You cannot specify theWHEN
clause for INSTEAD
OF
triggers.The trigger body is a CALL
procedure or a PL/SQL block that can include SQL and PL/SQL statements. The CALL
procedure can be either a PL/SQL or a Java procedure that is encapsulated in a PL/SQL wrapper. These statements are run if the triggering statement is entered and if the trigger restriction (if included) evaluates to TRUE
.
The trigger body for row triggers has some special constructs that can be included in the code of the PL/SQL block: correlation names and the REFERENCEING
option, and the conditional predicates INSERTING
, DELETING
, and UPDATING
.
Note:
TheINSERTING
, DELETING
, and UPDATING
conditional predicates cannot be used for the CALL
procedures; they can only be used in a PL/SQL block.Example: Monitoring Logons with a Trigger
Note:
You may need to set up data structures similar to the following for certain examples to work:CONNECT system/manager GRANT ADMINISTER DATABASE TRIGGER TO scott; CONNECT scott/tiger CREATE TABLE audit_table ( seq number, user_at VARCHAR2(10), time_now DATE, term VARCHAR2(10), job VARCHAR2(10), proc VARCHAR2(10), enum NUMBER);
CREATE OR REPLACE PROCEDURE foo (c VARCHAR2) AS BEGIN INSERT INTO Audit_table (user_at) VALUES(c); END; CREATE OR REPLACE TRIGGER logontrig AFTER LOGON ON DATABASE -- Just call an existing procedure. The ORA_LOGIN_USER is a function -- that returns information about the event that fired the trigger. CALL foo (ora_login_user) /
Example: Calling a Java Procedure from a Trigger
Although triggers are declared using PL/SQL, they can call procedures in other languages, such as Java:
CREATE OR REPLACE PROCEDURE Before_delete (Id IN NUMBER, Ename VARCHAR2) IS language Java name 'thjvTriggers.beforeDelete (oracle.sql.NUMBER, oracle.sql.CHAR)'; CREATE OR REPLACE TRIGGER Pre_del_trigger BEFORE DELETE ON Tab FOR EACH ROW CALL Before_delete (:old.Id, :old.Ename) /
The corresponding Java file is thjvTriggers.java
:
import java.sql.* import java.io.* import oracle.sql.* import oracle.oracore.* public class thjvTriggers { public state void beforeDelete (NUMBER old_id, CHAR old_name) Throws SQLException, CoreException { Connection conn = JDBCConnection.defaultConnection(); Statement stmt = conn.CreateStatement(); String sql = "insert into logtab values ("+ old_id.intValue() +", '"+ old_ename.toString() + ", BEFORE DELETE'); stmt.executeUpdate (sql); stmt.close(); return; } }
Within a trigger body of a row trigger, the PL/SQL code and SQL statements have access to the old and new column values of the current row affected by the triggering statement. Two correlation names exist for every column of the table being modified: one for the old column value, and one for the new column value. Depending on the type of triggering statement, certain correlation names might not have any meaning.
A trigger fired by an INSERT
statement has meaningful access to new column values only. Because the row is being created by the INSERT
, the old values are null.
A trigger fired by an UPDATE
statement has access to both old and new column values for both BEFORE
and AFTER
row triggers.
A trigger fired by a DELETE
statement has meaningful access to :old
column values only. Because the row no longer exists after the row is deleted, the :new
values are NULL
. However, you cannot modify :new
values because ORA
-4084
is raised if you try to modify :new
values.
The new column values are referenced using the new
qualifier before the column name, while the old column values are referenced using the old
qualifier before the column name. For example, if the triggering statement is associated with the Emp_tab
table (with the columns SAL
, COMM
, and so on), then you can include statements in the trigger body. For example:
IF :new.Sal > 10000 ... IF :new.Sal < :old.Sal ...
Old and new values are available in both BEFORE
and AFTER
row triggers. A new
column value can be assigned in a BEFORE
row trigger, but not in an AFTER
row trigger (because the triggering statement takes effect before an AFTER
row trigger is fired). If a BEFORE
row trigger changes the value of new
.column
, then an AFTER
row trigger fired by the same statement sees the change assigned by the BEFORE
row trigger.
Correlation names can also be used in the Boolean expression of a WHEN
clause. A colon (:
) must precede the old
and new
qualifiers when they are used in a trigger body, but a colon is not allowed when using the qualifiers in the WHEN
clause or the REFERENCING
option.
You can treat LOB columns the same as other columns, using regular SQL and PL/SQL functions with CLOB
columns, and calls to the DBMS_LOB
package with BLOB
columns:
drop table tab1; create table tab1 (c1 clob); insert into tab1 values ('<h1>HTML Document Fragment</h1><p>Some text.'); create or replace trigger trg1 before update on tab1 for each row begin dbms_output.put_line('Old value of CLOB column: '||:OLD.c1); dbms_output.put_line('Proposed new value of CLOB column: '||:NEW.c1); -- Previously, we couldn't change the new value for a LOB. -- Now, we can replace it, or construct a new value using SUBSTR, INSTR... -- operations for a CLOB, or DBMS_LOB calls for a BLOB. :NEW.c1 := :NEW.c1 || to_clob('<hr><p>Standard footer paragraph.'); dbms_output.put_line('Final value of CLOB column: '||:NEW.c1); end; / set serveroutput on; update tab1 set c1 = '<h1>Different Document Fragment</h1><p>Different text.'; select * from tab1;
In the case of INSTEAD
OF
triggers on nested table view columns, the new
and old
qualifiers correspond to the new and old nested table elements. The parent row corresponding to this nested table element can be accessed using the parent
qualifier. The parent correlation name is meaningful and valid only inside a nested table trigger.
The REFERENCING
option can be specified in a trigger body of a row trigger to avoid name conflicts among the correlation names and tables that might be named old
or new
. Because this is rare, this option is infrequently used.
For example, assume you have a table named new
with columns field1
(number) and field2
(character). The following CREATE
TRIGGER
example shows a trigger associated with the new
table that can use correlation names and avoid naming conflicts between the correlation names and the table name:
Note:
You may need to set up the following data structures for certain examples to work:CREATE TABLE new ( field1 NUMBER, field2 VARCHAR2(20));
CREATE OR REPLACE TRIGGER Print_salary_changes BEFORE UPDATE ON new REFERENCING new AS Newest FOR EACH ROW BEGIN :Newest.Field2 := TO_CHAR (:newest.field1); END;
Notice that the new
qualifier is renamed to newest
using the REFERENCING
option, and it is then used in the trigger body.
If more than one type of DML operation can fire a trigger (for example, ON
INSERT
OR
DELETE
OR
UPDATE
OF
Emp_tab
), the trigger body can use the conditional predicates INSERTING
, DELETING
, and UPDATING
to check which type of statement fire the trigger.
Within the code of the trigger body, you can execute blocks of code depending on the kind of DML operation fired the trigger:
IF INSERTING THEN ... END IF; IF UPDATING THEN ... END IF;
The first condition evaluates to TRUE
only if the statement that fired the trigger is an INSERT
statement; the second condition evaluates to TRUE
only if the statement that fired the trigger is an UPDATE
statement.
In an UPDATE
trigger, a column name can be specified with an UPDATING
conditional predicate to determine if the named column is being updated. For example, assume a trigger is defined as the following:
CREATE OR REPLACE TRIGGER ... ... UPDATE OF Sal, Comm ON Emp_tab ... BEGIN ... IF UPDATING ('SAL') THEN ... END IF; END;
The code in the THEN
clause runs only if the triggering UPDATE
statement updates the SAL
column. This way, the trigger can minimize its overhead when the column of interest is not being changed.
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). Therefore, a trigger body can prevent the execution of the triggering statement by raising an exception. User-defined exceptions are commonly used in triggers that enforce complex security authorizations or integrity constraints.
The only exception to this is when the event under consideration is database STARTUP
, SHUTDOWN
, or LOGIN
when the user logging in is SYSTEM
. In these scenarios, only the trigger action is rolled back.
You can use the OBJECT_VALUE
pseudocolumn in a trigger on an object table since 10g Release 1 (10.1). OBJECT_VALUE
means the object as a whole. This is one example of its use. You can also call a PL/SQL function with OBJECT_VALUE
as the datatype of an IN
formal parameter.
Here is an example of the use of OBJECT_VALUE
in a trigger. To keep track of updates to values in an object table tbl
, a history table, tbl_history
, is also created in the following example. For tbl
, the values 1 through 5 are inserted into n
, while m
is kept at 0. The trigger is a row-level trigger that executes once for each row affected by a DML statement. The trigger causes the old and new values of the object t
in tbl
to be written in tbl_history
when tbl
is updated. These old and new values are :OLD.OBJECT_VALUE
and :NEW.OBJECT_VALUE
. An update of the table tbl
is done (each value of n
is increased by 1). A select from the history table to check that the trigger works is then shown at the end of the example:
CREATE OR REPLACE TYPE t AS OBJECT (n NUMBER, m NUMBER) / CREATE TABLE tbl OF t / BEGIN FOR j IN 1..5 LOOP INSERT INTO tbl VALUES (t(j, 0)); END LOOP; END; / CREATE TABLE tbl_history ( d DATE, old_obj t, new_obj t) / CREATE OR REPLACE TRIGGER Tbl_Trg AFTER UPDATE ON tbl FOR EACH ROW BEGIN INSERT INTO tbl_history (d, old_obj, new_obj) VALUES (SYSDATE, :OLD.OBJECT_VALUE, :NEW.OBJECT_VALUE); END Tbl_Trg; / -------------------------------------------------------------------------------- UPDATE tbl SET tbl.n = tbl.n+1 / BEGIN FOR j IN (SELECT d, old_obj, new_obj FROM tbl_history) LOOP Dbms_Output.Put_Line ( j.d|| ' -- old: '||j.old_obj.n||' '||j.old_obj.m|| ' -- new: '||j.new_obj.n||' '||j.new_obj.m); END LOOP; END; /
The result of the select shows that the values of the column n
have been all increased by 1. The value of m
remains 0. The output of the select is:
23-MAY-05 -- old: 1 0 -- new: 2 0 23-MAY-05 -- old: 2 0 -- new: 3 0 23-MAY-05 -- old: 3 0 -- new: 4 0 23-MAY-05 -- old: 4 0 -- new: 5 0 23-MAY-05 -- old: 5 0 -- new: 6 0
A trigger that accesses a remote site cannot do remote exception handling if the network link is unavailable. For example:
CREATE OR REPLACE TRIGGER Example AFTER INSERT ON Emp_tab FOR EACH ROW BEGIN INSERT INTO Emp_tab@Remote -- <- compilation fails here VALUES ('x'); -- when dblink is inaccessible EXCEPTION WHEN OTHERS THEN INSERT INTO Emp_log VALUES ('x'); END;
A trigger is compiled when it is created. 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.
Because stored procedures are stored in a compiled form, the work-around for the previous example is as follows:
CREATE OR REPLACE TRIGGER Example AFTER INSERT ON Emp_tab FOR EACH ROW BEGIN Insert_row_proc; END; CREATE OR REPLACE PROCEDURE Insert_row_proc AS BEGIN INSERT INTO Emp_tab@Remote VALUES ('x'); EXCEPTION WHEN OTHERS THEN INSERT INTO Emp_log VALUES ('x'); END;
The trigger in this example compiles successfully and calls the stored procedure, which already has a validated statement for accessing the remote database; thus, when the remote INSERT
statement fails because the link is down, the exception is caught.
Coding triggers requires some restrictions that are not required for standard PL/SQL blocks. The following sections discuss these restrictions.
Maximum Trigger Size
The size of a trigger cannot be more than 32K.
SQL Statements Allowed in Trigger Bodies
The body of a trigger can contain DML SQL statements. It can also contain SELECT
statements, but they must be SELECT
... INTO
... statements or the SELECT
statement in the definition of a cursor.
DDL statements are not allowed in the body of a trigger. Also, no transaction control statements are allowed in a trigger. ROLLBACK
, COMMIT
, and SAVEPOINT
cannot be used.For system triggers, {CREATE
/ALTER
/DROP
} TABLE
statements and ALTER
...COMPILE
are allowed.
Note:
A procedure called by a trigger cannot run the previous transaction control statements, because the procedure runs within the context of the trigger body.Statements inside a trigger can reference remote schema objects. However, pay special attention when calling remote procedures from within a local trigger. If a timestamp or signature mismatch is found during execution of the trigger, then the remote procedure is not run, and the trigger is invalidated.
Trigger Restrictions on LONG and LONG RAW Datatypes
LONG
and LONG
RAW
datatypes in triggers are subject to the following restrictions:
A SQL statement within a trigger can insert data into a column of LONG
or LONG
RAW
datatype.
If data from a LONG
or LONG
RAW
column can be converted to a constrained datatype (such as CHAR
and VARCHAR2
), then a LONG
or LONG
RAW
column can be referenced in a SQL statement within a trigger. The maximum length for these datatypes is 32000 bytes.
Variables cannot be declared using the LONG
or LONG
RAW
datatypes.
:NEW
and :PARENT
cannot be used with LONG
or LONG
RAW
columns.
Trigger Restrictions on Mutating Tables
A mutating table is a table that is being modified by an UPDATE
, DELETE
, or INSERT
statement, or a table that might be updated by the effects of a DELETE
CASCADE
constraint.
The session that issued the triggering statement cannot query or modify a mutating table. This restriction prevents a trigger from seeing an inconsistent set of data.
This restriction applies to all triggers that use the FOR EACH ROW
clause. Views being modified in INSTEAD
OF
triggers are not considered mutating.
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.
Consider the following trigger:
CREATE OR REPLACE TRIGGER Emp_count AFTER DELETE ON Emp_tab FOR EACH ROW DECLARE n INTEGER; BEGIN SELECT COUNT(*) INTO n FROM Emp_tab; DBMS_OUTPUT.PUT_LINE(' There are now ' || n || ' employees.'); END;
If the following SQL statement is entered:
DELETE FROM Emp_tab WHERE Empno = 7499;
An error is returned because the table is mutating when the row is deleted:
ORA-04091: table SCOTT.Emp_tab is mutating, trigger/function may not see it
If you delete the line "FOR
EACH
ROW
" from the trigger, it becomes a statement trigger which is not subject to this restriction, and the trigger.
If you need to update a mutating table, you could bypass these restrictions by using a temporary table, a PL/SQL table, or a package variable. For example, in place of a single AFTER
row trigger that updates the original table, resulting in a mutating table error, you might use two triggers—an AFTER
row trigger that updates a temporary table, and an AFTER
statement trigger that updates the original table with the values from the temporary table.
Declarative integrity constraints are checked at various times with respect to row triggers.
See Also:
Oracle Database Concepts for information about the interaction of triggers and integrity constraintsBecause 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. These restrictions are also not enforced among tables in the same database that are connected by loop-back database links. 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.
Restrictions on Mutating Tables Relaxed
The mutating error, discussed earlier in this section, still prevents the trigger from reading or modifying the table that the parent statement is modifying. However, starting in Oracle Database Release 8.1, a delete against the parent table causes before/after statement triggers to be fired once. That way, you can create triggers (just not row triggers) to read and modify the parent and child tables.
This allows most foreign key constraint actions to be implemented through their obvious after-row trigger, providing the constraint is not self-referential. Update cascade, update set null, update set default, delete set default, inserting a missing parent, and maintaining a count of children can all be implemented easily. For example, this is an implementation of update cascade:
create table p (p1 number constraint ppk primary key); create table f (f1 number constraint ffk references p); create trigger pt after update on p for each row begin update f set f1 = :new.p1 where f1 = :old.p1; end; /
This implementation requires care for multirow updates. 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:
update p set p1 = p1+1;
The statement first updates (1) to (2) in p, and the trigger updates (1) to (2) in f, leaving two rows of value (2) in f. Then the statement updates (2) to (3) in p, and the trigger updates both rows of value (2) to (3) in f. Finally, the statement updates (3) to (4) in p, and the trigger updates all three rows in f from (3) to (4). The relationship of the data in p and f is lost.
To avoid this problem, you must forbid multirow updates to p that change the primary key and reuse existing primary key values. It could also be solved by tracking which foreign key values have already been updated, then modifying the trigger so that no row is updated twice.
That is the only problem with this technique for foreign key updates. The trigger cannot miss rows that have been changed but not committed by another transaction, because the foreign key constraint guarantees that no matching foreign key rows are locked before the after-row trigger is called.
Depending on the event, different event attribute functions are available. For example, certain DDL operations may not be allowed on DDL events. Check "Event Attribute Functions" before using an event attribute function, because its effects might be undefined rather than producing an error condition.
Only committed triggers are fired. 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.
For example, if you execute the following SQL statement:
CREATE OR REPLACE TRIGGER my_trigger AFTER CREATE ON DATABASE BEGIN null; END;
Then, trigger my_trigger
is not fired after the creation of my_trigger
. Oracle Database does not fire a trigger that is not committed.
Foreign Function Callouts
All restrictions on foreign function callouts also apply.
The following statement, inside a trigger, returns the owner of the trigger, not the name of user who is updating the table:
SELECT Username FROM USER_USERS;
To create a trigger in your schema, you must have the CREATE
TRIGGER
system privilege, and either:
Own the table specified in the triggering statement, or
Have the ALTER
privilege for the table in the triggering statement, or
Have the ALTER
ANY
TABLE
system privilege
To create a trigger in another user's schema, or to reference a table in another schema from a trigger in your schema, you must have the CREATE
ANY
TRIGGER
system privilege. With this privilege, the trigger can be created in any schema and can be associated with any user's table. In addition, the user creating the trigger must also have EXECUTE
privilege on the referenced procedures, functions, or packages.
To create a trigger on DATABASE
, you must have the ADMINISTER
DATABASE
TRIGGER
privilege. If this privilege is later revoked, then you can drop the trigger, but not alter it.
The object privileges to the schema objects referenced in the trigger body must be granted to the trigger owner explicitly (not through a role). The statements in the trigger body operate under the privilege domain of the trigger owner, not the privilege domain of the user issuing the triggering statement. This is similar to the privilege model for stored procedures.
Triggers are similar to PL/SQL anonymous blocks with the addition of the :new
and :old
capabilities, but their compilation is different. A PL/SQL anonymous block is compiled each time it is loaded into memory. Compilation involves three stages:
Syntax checking: PL/SQL syntax is checked, and a parse tree is generated.
Semantic checking: Type checking and further processing on the parse tree.
Triggers, in contrast, are fully compiled when the CREATE
TRIGGER
statement is entered, and the pcode is stored in the data dictionary. Hence, firing the trigger no longer requires the opening of a shared cursor to run the trigger action. Instead, the trigger is executed directly.
If errors occur during the compilation of a trigger, then the trigger is still created. If a DML statement fires this trigger, then the DML statement fails. (Runtime that trigger errors always cause the DML statement to fail.) You can use the SHOW
ERRORS
statement in SQL*Plus or Enterprise Manager to see any compilation errors when you create a trigger, or you can SELECT
the errors from the USER_ERRORS
view.
Compiled triggers have dependencies. They become invalid if a depended-on object, such as a stored procedure or function called from the trigger body, is modified. Triggers that are invalidated for dependency reasons are recompiled when next invoked.
You can examine the ALL_DEPENDENCIES
view to see the dependencies for a trigger. For example, the following statement shows the dependencies for the triggers in the SCOTT
schema:
SELECT NAME, REFERENCED_OWNER, REFERENCED_NAME, REFERENCED_TYPE FROM ALL_DEPENDENCIES WHERE OWNER = 'SCOTT' and TYPE = 'TRIGGER';
Triggers may depend on other functions or packages. If the function or package specified in the trigger is dropped, then the trigger is marked invalid. An attempt is made to validate the trigger on occurrence of the event. If the trigger cannot be validated successfully, then it is marked VALID
WITH
ERRORS
, and the event fails.
Note:
There is an exception for STARTUP
events: STARTUP
events succeed even if the trigger fails. There are also exceptions for SHUTDOWN
events and for LOGON
events if you login as SYSTEM
.
Because the DBMS_AQ
package is used to enqueue a message, dependency between triggers and queues cannot be maintained.
Use the ALTER
TRIGGER
statement to recompile a trigger manually. For example, the following statement recompiles the PRINT_SALARY_CHANGES
trigger:
ALTER TRIGGER Print_salary_changes COMPILE;
To recompile a trigger, you must own the trigger or have the ALTER
ANY
TRIGGER
system privilege.
Like a stored procedure, a trigger cannot be explicitly altered: It must be replaced with a new definition. (The ALTER
TRIGGER
statement is used only to recompile, enable, or disable a trigger.)
When replacing a trigger, you must include the OR
REPLACE
option in the CREATE
TRIGGER
statement. The OR
REPLACE
option is provided to allow a new version of an existing trigger to replace the older version, without affecting any grants made for the original version of the trigger.
Alternatively, the trigger can be dropped using the DROP
TRIGGER
statement, and you can rerun the CREATE
TRIGGER
statement.
To drop a trigger, the trigger must be in your schema, or you must have the DROP
ANY
TRIGGER
system privilege.
You can debug a trigger using the same facilities available for stored procedures.
See Also:
"Debugging Stored Procedures"A trigger can be in one of two distinct modes:
Enabled. An enabled trigger executes its trigger body if a triggering statement is entered and the trigger restriction (if any) evaluates to TRUE
.
Disabled. A disabled trigger does not execute its trigger body, even if a triggering statement is entered and the trigger restriction (if any) evaluates to TRUE
.
By default, a trigger is automatically enabled when it is created; however, it can later be disabled. After you have completed the task that required the trigger to be disabled, re-enable the trigger, so that it fires when appropriate.
Enable a disabled trigger using the ALTER
TRIGGER
statement with the ENABLE
option. To enable the disabled trigger named REORDER
of the INVENTORY
table, enter the following statement:
ALTER TRIGGER Reorder ENABLE;
All triggers defined for a specific table can be enabled with one statement using the ALTER
TABLE
statement with the ENABLE
clause with the ALL
TRIGGERS
option. For example, to enable all triggers defined for the INVENTORY
table, enter the following statement:
ALTER TABLE Inventory ENABLE ALL TRIGGERS;
You might temporarily disable a trigger if:
An object it references is not available.
You need to perform a large data load, and you want it to proceed quickly without firing triggers.
You are reloading data.
By default, triggers are enabled when first created. Disable a trigger using the ALTER
TRIGGER
statement with the DISABLE
option.
For example, to disable the trigger named REORDER
of the INVENTORY
table, enter the following statement:
ALTER TRIGGER Reorder DISABLE;
All triggers associated with a table can be disabled with one statement using the ALTER
TABLE
statement with the DISABLE
clause and the ALL
TRIGGERS
option. For example, to disable all triggers defined for the INVENTORY
table, enter the following statement:
ALTER TABLE Inventory DISABLE ALL TRIGGERS;
The following data dictionary views reveal information about triggers:
USER_TRIGGERS
ALL_TRIGGERS
DBA_TRIGGERS
The new column, BASE_OBJECT_TYPE
, specifies whether the trigger is based on DATABASE
, SCHEMA
, table, or view. The old column, TABLE_NAME
, is null if the base object is not table or view.
The column ACTION_TYPE
specifies whether the trigger is a call type trigger or a PL/SQL trigger.
The column TRIGGER_TYPE
includes two additional values: BEFORE
EVENT
and AFTER
EVENT
, applicable only to system events.
The column TRIGGERING_EVENT
includes all system and DML events.
See Also:
Oracle Database Reference for a complete description of these data dictionary viewsFor example, assume the following statement was used to create the REORDER
trigger:
Caution:
You may need to set up data structures for certain examples to work:CREATE OR REPLACE TRIGGER Reorder AFTER UPDATE OF Parts_on_hand ON Inventory FOR EACH ROW WHEN(new.Parts_on_hand < new.Reorder_point) DECLARE x NUMBER; BEGIN SELECT COUNT(*) INTO x FROM Pending_orders WHERE Part_no = :new.Part_no; IF x = 0 THEN INSERT INTO Pending_orders VALUES (:new.Part_no, :new.Reorder_quantity, sysdate); END IF; END;
The following two queries return information about the REORDER
trigger:
SELECT Trigger_type, Triggering_event, Table_name FROM USER_TRIGGERS WHERE Trigger_name = 'REORDER'; TYPE TRIGGERING_STATEMENT TABLE_NAME ---------------- -------------------------- ------------ AFTER EACH ROW UPDATE INVENTORY SELECT Trigger_body FROM USER_TRIGGERS WHERE Trigger_name = 'REORDER'; TRIGGER_BODY -------------------------------------------- DECLARE x NUMBER; BEGIN SELECT COUNT(*) INTO x FROM Pending_orders WHERE Part_no = :new.Part_no; IF x = 0 THEN INSERT INTO Pending_orders VALUES (:new.Part_no, :new.Reorder_quantity, sysdate); END IF; END;
You can use triggers in a number of ways to customize information management in Oracle Database. For example, triggers are commonly used to:
Provide sophisticated auditing
Prevent invalid transactions
Enforce referential integrity (either those actions not supported by declarative integrity constraints or across nodes in a distributed database)
Enforce complex business rules
Enforce complex security authorizations
Provide transparent event logging
Automatically generate derived column values
Enable building complex views that are updatable
Track system events
This section provides an example of each of these trigger applications. These examples are not meant to be used exactly as written: They are provided to assist you in designing your own triggers.
Auditing with Triggers: Example
Triggers are commonly used to supplement the built-in auditing features of Oracle Database. Although triggers can be written to record information similar to that recorded by the AUDIT
statement, triggers should be used only when more detailed audit information is required. For example, use triggers to provide value-based auditing for each row.
Sometimes, the AUDIT
statement is considered a security audit facility, while triggers can provide financial audit facility.
When deciding whether to create a trigger to audit database activity, consider what Oracle Database's auditing features provide, compared to auditing defined by triggers, as shown in Table 9-1.
Table 9-1 Comparison of Built-in Auditing and Trigger-Based Auditing
Audit Feature | Description |
---|---|
DML and DDL Auditing |
Standard auditing options permit auditing of DML and DDL statements regarding all types of schema objects and structures. Comparatively, triggers permit auditing of DML statements entered against tables, and DDL auditing at |
Centralized Audit Trail |
All database audit information is recorded centrally and automatically using the auditing features of Oracle Database. |
Declarative Method |
Auditing features enabled using the standard Oracle Database features are easier to declare and maintain, and less prone to errors, when compared to auditing functions defined by triggers. |
Auditing Options can be Audited |
Any changes to existing auditing options can also be audited to guard against malicious database activity. |
Session and Execution time Auditing |
Using the database auditing features, records can be generated once every time an audited statement is entered ( |
Auditing of Unsuccessful Data Access |
Database auditing can be set to audit when unsuccessful data access occurs. However, unless autonomous transactions are used, any audit information generated by a trigger is rolled back if the triggering statement is rolled back. For more information on autonomous transactions, refer to Oracle Database Concepts. |
Sessions can be Audited |
Connections and disconnections, as well as session activity (physical I/Os, logical I/Os, deadlocks, and so on), can be recorded using standard database auditing. |
When using triggers to provide sophisticated auditing, AFTER
triggers are normally used. By using AFTER
triggers, auditing information is recorded after the triggering statement is subjected to any applicable integrity constraints, preventing cases where the audit processing is carried out unnecessarily for statements that generate exceptions to integrity constraints.
Choosing between AFTER
row and AFTER
statement triggers depends on the information being audited. For example, row triggers provide value-based auditing for each table row. Triggers can also require the user to supply a "reason code" for issuing the audited SQL statement, which can be useful in both row and statement-level auditing situations.
The following example demonstrates a trigger that audits modifications to the Emp_tab
table for each row. It requires that a "reason code" be stored in a global package variable before the update. This shows how triggers can be used to provide value-based auditing and how to use public package variables.
Note:
You may need to set up the following data structures for the examples to work:CREATE OR REPLACE PACKAGE Auditpackage AS Reason VARCHAR2(10); PROCEDURE Set_reason(Reason VARCHAR2); END; CREATE TABLE Emp99 ( Empno NOT NULL NUMBER(4), Ename VARCHAR2(10), Job VARCHAR2(9), Mgr NUMBER(4), Hiredate DATE, Sal NUMBER(7,2), Comm NUMBER(7,2), Deptno NUMBER(2), Bonus NUMBER, Ssn NUMBER, Job_classification NUMBER); CREATE TABLE Audit_employee ( Oldssn NUMBER, Oldname VARCHAR2(10), Oldjob VARCHAR2(2), Oldsal NUMBER, Newssn NUMBER, Newname VARCHAR2(10), Newjob VARCHAR2(2), Newsal NUMBER, Reason VARCHAR2(10), User1 VARCHAR2(10), Systemdate DATE);
CREATE OR REPLACE TRIGGER Audit_employee AFTER INSERT OR DELETE OR UPDATE ON Emp99 FOR EACH ROW BEGIN /* AUDITPACKAGE is a package with a public package variable REASON. REASON could be set by the application by a command such as EXECUTE AUDITPACKAGE.SET_REASON(reason_string). Note that a package variable has state for the duration of a session and that each session has a separate copy of all package variables. */ IF Auditpackage.Reason IS NULL THEN Raise_application_error(-20201, 'Must specify reason' || ' with AUDITPACKAGE.SET_REASON(Reason_string)'); END IF; /* If the preceding conditional evaluates to TRUE, the user-specified error number and message is raised, the trigger stops execution, and the effects of the triggering statement are rolled back. Otherwise, a new row is inserted into the predefined auditing table named AUDIT_EMPLOYEE containing the existing and new values of the Emp_tab table and the reason code defined by the REASON variable of AUDITPACKAGE. Note that the "old" values are NULL if triggering statement is an INSERT and the "new" values are NULL if the triggering statement is a DELETE. */ INSERT INTO Audit_employee VALUES (:old.Ssn, :old.Ename, :old.Job_classification, :old.Sal, :new.Ssn, :new.Ename, :new.Job_classification, :new.Sal, auditpackage.Reason, User, Sysdate ); END;
Optionally, you can also set the reason code back to NULL
if you wanted to force the reason code to be set for every update. The following simple AFTER
statement trigger sets the reason code back to NULL
after the triggering statement is run:
CREATE OR REPLACE TRIGGER Audit_employee_reset AFTER INSERT OR DELETE OR UPDATE ON Emp_tab BEGIN auditpackage.set_reason(NULL); END;
Notice that the previous two triggers are both fired by the same type of SQL statement. However, the AFTER
row trigger is fired once for each row of the table affected by the triggering statement, while the AFTER
statement trigger is fired only once after the triggering statement execution is completed.
This next trigger also uses triggers to do auditing. It tracks changes made to the Emp_tab
table and stores this information in AUDIT_TABLE
and AUDIT_TABLE_VALUES
.
Note:
You may need to set up the following data structures for the example to work:CREATE TABLE Audit_table ( Seq NUMBER, User_at VARCHAR2(10), Time_now DATE, Term VARCHAR2(10), Job VARCHAR2(10), Proc VARCHAR2(10), enum NUMBER); CREATE SEQUENCE Audit_seq; CREATE TABLE Audit_table_values ( Seq NUMBER, Dept NUMBER, Dept1 NUMBER, Dept2 NUMBER);
CREATE OR REPLACE TRIGGER Audit_emp AFTER INSERT OR UPDATE OR DELETE ON Emp_tab FOR EACH ROW DECLARE Time_now DATE; Terminal CHAR(10); BEGIN -- get current time, and the terminal of the user: Time_now := SYSDATE; Terminal := USERENV('TERMINAL'); -- record new employee primary key IF INSERTING THEN INSERT INTO Audit_table VALUES (Audit_seq.NEXTVAL, User, Time_now, Terminal, 'Emp_tab', 'INSERT', :new.Empno); -- record primary key of the deleted row: ELSIF DELETING THEN INSERT INTO Audit_table VALUES (Audit_seq.NEXTVAL, User, Time_now, Terminal, 'Emp_tab', 'DELETE', :old.Empno); -- for updates, record the primary key -- of the row being updated: ELSE INSERT INTO Audit_table VALUES (audit_seq.NEXTVAL, User, Time_now, Terminal, 'Emp_tab', 'UPDATE', :old.Empno); -- and for SAL and DEPTNO, record old and new values: IF UPDATING ('SAL') THEN INSERT INTO Audit_table_values VALUES (Audit_seq.CURRVAL, 'SAL', :old.Sal, :new.Sal); ELSIF UPDATING ('DEPTNO') THEN INSERT INTO Audit_table_values VALUES (Audit_seq.CURRVAL, 'DEPTNO', :old.Deptno, :new.DEPTNO); END IF; END IF; END;
Integrity Constraints and Triggers: Examples
Triggers and declarative integrity constraints can both be used to constrain data input. However, triggers and integrity constraints have significant differences.
Declarative integrity constraints are statements about the database that are always true. A constraint applies to existing data in the table and any statement that manipulates the table.
Triggers constrain what a transaction can do. A trigger does not apply to data loaded before the definition of the trigger; therefore, it is not known if all data in a table conforms to the rules established by an associated trigger.
Although triggers can be written to enforce many of the same rules supported by Oracle Database's declarative integrity constraint features, triggers should only be used to enforce complex business rules that cannot be defined using standard integrity constraints. The declarative integrity constraint features provided with Oracle Database offer the following advantages when compared to constraints defined by triggers:
Centralized integrity checks. All points of data access must adhere to the global set of rules defined by the integrity constraints corresponding to each schema object.
Declarative method. Constraints defined using the standard integrity constraint features are much easier to write and are less prone to errors, when compared with comparable constraints defined by triggers.
While most aspects of data integrity can be defined and enforced using declarative integrity constraints, triggers can be used to enforce complex business constraints not definable using declarative integrity constraints. For example, triggers can be used to enforce:
UPDATE
SET
NULL
, and UPDATE
and DELETE
SET
DEFAULT
referential actions.
Referential integrity when the parent and child tables are on different nodes of a distributed database.
Complex check constraints not definable using the expressions allowed in a CHECK
constraint.
Referential Integrity Using Triggers
There are many cases where referential integrity can be enforced using triggers. Note, however, you should only use triggers when there is no declarative support for the action you are performing.
When using triggers to maintain referential integrity, declare the PRIMARY
(or UNIQUE
) KEY
constraint in the parent table. If referential integrity is being maintained between a parent and child table in the same database, then you can also declare the foreign key in the child table, but disable it; this prevents the corresponding PRIMARY
KEY
constraint from being dropped (unless the PRIMARY
KEY
constraint is explicitly dropped with the CASCADE
option).
To maintain referential integrity using triggers:
A trigger must be defined for the child table that guarantees values inserted or updated in the foreign key correspond to values in the parent key.
One or more triggers must be defined for the parent table. These triggers guarantee the desired referential action (RESTRICT
, CASCADE
, or SET
NULL
) for values in the foreign key when values are updated or deleted in the parent key. No action is required for inserts into the parent table (no dependent foreign keys exist).
The following sections provide examples of the triggers necessary to enforce referential integrity. The Emp_tab
and Dept_tab
table relationship is used in these examples.
Several of the triggers include statements that lock rows (SELECT
... FOR
UPDATE
). This operation is necessary to maintain concurrency as the rows are being processed.
Foreign Key Trigger for Child Table The following trigger guarantees that before an INSERT
or UPDATE
statement affects a foreign key value, the corresponding value exists in the parent key. The mutating table exception included in the following example allows this trigger to be used with the UPDATE_SET_DEFAULT
and UPDATE_CASCADE
triggers. This exception can be removed if this trigger is used alone.
CREATE OR REPLACE TRIGGER Emp_dept_check BEFORE INSERT OR UPDATE OF Deptno ON Emp_tab FOR EACH ROW WHEN (new.Deptno IS NOT NULL) -- Before a row is inserted, or DEPTNO is updated in the Emp_tab -- table, fire this trigger to verify that the new foreign -- key value (DEPTNO) is present in the Dept_tab table. DECLARE Dummy INTEGER; -- to be used for cursor fetch Invalid_department EXCEPTION; Valid_department EXCEPTION; Mutating_table EXCEPTION; PRAGMA EXCEPTION_INIT (Mutating_table, -4091); -- Cursor used to verify parent key value exists. If -- present, lock parent key's row so it can't be -- deleted by another transaction until this -- transaction is committed or rolled back. CURSOR Dummy_cursor (Dn NUMBER) IS SELECT Deptno FROM Dept_tab WHERE Deptno = Dn FOR UPDATE OF Deptno; BEGIN OPEN Dummy_cursor (:new.Deptno); FETCH Dummy_cursor INTO Dummy; -- Verify parent key. If not found, raise user-specified -- error number and message. If found, close cursor -- before allowing triggering statement to complete: IF Dummy_cursor%NOTFOUND THEN RAISE Invalid_department; ELSE RAISE valid_department; END IF; CLOSE Dummy_cursor; EXCEPTION WHEN Invalid_department THEN CLOSE Dummy_cursor; Raise_application_error(-20000, 'Invalid Department' || ' Number' || TO_CHAR(:new.deptno)); WHEN Valid_department THEN CLOSE Dummy_cursor; WHEN Mutating_table THEN NULL; END;
UPDATE and DELETE RESTRICT Trigger for Parent Table The following trigger is defined on the DEPT_TAB
table to enforce the UPDATE
and DELETE
RESTRICT
referential action on the primary key of the DEPT_TAB
table:
CREATE OR REPLACE TRIGGER Dept_restrict BEFORE DELETE OR UPDATE OF Deptno ON Dept_tab FOR EACH ROW -- Before a row is deleted from Dept_tab or the primary key -- (DEPTNO) of Dept_tab is updated, check for dependent -- foreign key values in Emp_tab; rollback if any are found. DECLARE Dummy INTEGER; -- to be used for cursor fetch Employees_present EXCEPTION; employees_not_present EXCEPTION; -- Cursor used to check for dependent foreign key values. CURSOR Dummy_cursor (Dn NUMBER) IS SELECT Deptno FROM Emp_tab WHERE Deptno = Dn; BEGIN OPEN Dummy_cursor (:old.Deptno); FETCH Dummy_cursor INTO Dummy; -- If dependent foreign key is found, raise user-specified -- error number and message. If not found, close cursor -- before allowing triggering statement to complete. IF Dummy_cursor%FOUND THEN RAISE Employees_present; -- dependent rows exist ELSE RAISE Employees_not_present; -- no dependent rows END IF; CLOSE Dummy_cursor; EXCEPTION WHEN Employees_present THEN CLOSE Dummy_cursor; Raise_application_error(-20001, 'Employees Present in' || ' Department ' || TO_CHAR(:old.DEPTNO)); WHEN Employees_not_present THEN CLOSE Dummy_cursor; END;
Caution:
This trigger does not work with self-referential tables (tables with both the primary/unique key and the foreign key). Also, this trigger does not allow triggers to cycle (such as, A fires B fires A).UPDATE and DELETE SET NULL Triggers for Parent Table: Example The following trigger is defined on the DEPT_TAB
table to enforce the UPDATE
and DELETE
SET
NULL
referential action on the primary key of the DEPT_TAB
table:
CREATE OR REPLACE TRIGGER Dept_set_null AFTER DELETE OR UPDATE OF Deptno ON Dept_tab FOR EACH ROW -- Before a row is deleted from Dept_tab or the primary key -- (DEPTNO) of Dept_tab is updated, set all corresponding -- dependent foreign key values in Emp_tab to NULL: BEGIN IF UPDATING AND :OLD.Deptno != :NEW.Deptno OR DELETING THEN UPDATE Emp_tab SET Emp_tab.Deptno = NULL WHERE Emp_tab.Deptno = :old.Deptno; END IF; END;
DELETE Cascade Trigger for Parent Table: Example The following trigger on the DEPT_TAB
table enforces the DELETE
CASCADE
referential action on the primary key of the DEPT_TAB
table:
CREATE OR REPLACE TRIGGER Dept_del_cascade AFTER DELETE ON Dept_tab FOR EACH ROW -- Before a row is deleted from Dept_tab, delete all -- rows from the Emp_tab table whose DEPTNO is the same as -- the DEPTNO being deleted from the Dept_tab table: BEGIN DELETE FROM Emp_tab WHERE Emp_tab.Deptno = :old.Deptno; END;
Note:
Typically, the code forDELETE
CASCADE
is combined with the code for UPDATE
SET
NULL
or UPDATE
SET
DEFAULT
to account for both updates and deletes.UPDATE Cascade Trigger for Parent Table: Example The following trigger ensures that if a department number is updated in the Dept_tab
table, then this change is propagated to dependent foreign keys in the Emp_tab
table:
-- Generate a sequence number to be used as a flag for -- determining if an update has occurred on a column: CREATE SEQUENCE Update_sequence INCREMENT BY 1 MAXVALUE 5000 CYCLE; CREATE OR REPLACE PACKAGE Integritypackage AS Updateseq NUMBER; END Integritypackage; CREATE OR REPLACE PACKAGE BODY Integritypackage AS END Integritypackage; -- create flag col: ALTER TABLE Emp_tab ADD Update_id NUMBER; CREATE OR REPLACE TRIGGER Dept_cascade1 BEFORE UPDATE OF Deptno ON Dept_tab DECLARE Dummy NUMBER; -- Before updating the Dept_tab table (this is a statement -- trigger), generate a new sequence number and assign -- it to the public variable UPDATESEQ of a user-defined -- package named INTEGRITYPACKAGE: BEGIN SELECT Update_sequence.NEXTVAL INTO Dummy FROM dual; Integritypackage.Updateseq := Dummy; END; CREATE OR REPLACE TRIGGER Dept_cascade2 AFTER DELETE OR UPDATE OF Deptno ON Dept_tab FOR EACH ROW -- For each department number in Dept_tab that is updated, -- cascade the update to dependent foreign keys in the -- Emp_tab table. Only cascade the update if the child row -- has not already been updated by this trigger: BEGIN IF UPDATING THEN UPDATE Emp_tab SET Deptno = :new.Deptno, Update_id = Integritypackage.Updateseq --from 1st WHERE Emp_tab.Deptno = :old.Deptno AND Update_id IS NULL; /* only NULL if not updated by the 3rd trigger fired by this same triggering statement */ END IF; IF DELETING THEN -- Before a row is deleted from Dept_tab, delete all -- rows from the Emp_tab table whose DEPTNO is the same as -- the DEPTNO being deleted from the Dept_tab table: DELETE FROM Emp_tab WHERE Emp_tab.Deptno = :old.Deptno; END IF; END; CREATE OR REPLACE TRIGGER Dept_cascade3 AFTER UPDATE OF Deptno ON Dept_tab BEGIN UPDATE Emp_tab SET Update_id = NULL WHERE Update_id = Integritypackage.Updateseq; END;
Note:
Because this trigger updates theEmp_tab
table, the Emp_dept_check
trigger, if enabled, is also fired. The resulting mutating table error is trapped by the Emp_dept_check
trigger. You should carefully test any triggers that require error trapping to succeed to ensure that they always work properly in your environment.Trigger for Complex Check Constraints: Example
Triggers can enforce integrity rules other than referential integrity. For example, this trigger performs a complex check before allowing the triggering statement to run.
Note:
You may need to set up the following data structures for the example to work:CREATE TABLE Salgrade ( Grade NUMBER, Losal NUMBER, Hisal NUMBER, Job_classification NUMBER)
CREATE OR REPLACE TRIGGER Salary_check BEFORE INSERT OR UPDATE OF Sal, Job ON Emp99 FOR EACH ROW DECLARE Minsal NUMBER; Maxsal NUMBER; Salary_out_of_range EXCEPTION; BEGIN /* Retrieve the minimum and maximum salary for the employee's new job classification from the SALGRADE table into MINSAL and MAXSAL: */ SELECT Minsal, Maxsal INTO Minsal, Maxsal FROM Salgrade WHERE Job_classification = :new.Job; /* If the employee's new salary is less than or greater than the job classification's limits, the exception is raised. The exception message is returned and the pending INSERT or UPDATE statement that fired the trigger is rolled back:*/ IF (:new.Sal < Minsal OR :new.Sal > Maxsal) THEN RAISE Salary_out_of_range; END IF; EXCEPTION WHEN Salary_out_of_range THEN Raise_application_error (-20300, 'Salary '||TO_CHAR(:new.Sal)||' out of range for ' ||'job classification '||:new.Job ||' for employee '||:new.Ename); WHEN NO_DATA_FOUND THEN Raise_application_error(-20322, 'Invalid Job Classification ' ||:new.Job_classification); END;
Complex Security Authorizations and Triggers: Example
Triggers are commonly used to enforce complex security authorizations for table data. Only use triggers to enforce complex security authorizations that cannot be defined using the database security features provided with Oracle Database. For example, a trigger can prohibit updates to salary data of the Emp_tab
table during weekends, holidays, and non-working hours.
When using a trigger to enforce a complex security authorization, it is best to use a BEFORE
statement trigger. Using a BEFORE
statement trigger has these benefits:
The security check is done before the triggering statement is allowed to run, so that no wasted work is done by an unauthorized statement.
The security check is performed only once for the triggering statement, not for each row affected by the triggering statement.
This example shows a trigger used to enforce security.
Note:
You may need to set up the following data structures for the example to work:CREATE TABLE Company_holidays (Day DATE);
CREATE OR REPLACE TRIGGER Emp_permit_changes BEFORE INSERT OR DELETE OR UPDATE ON Emp99 DECLARE Dummy INTEGER; Not_on_weekends EXCEPTION; Not_on_holidays EXCEPTION; Non_working_hours EXCEPTION; BEGIN /* check for weekends: */ IF (TO_CHAR(Sysdate, 'DY') = 'SAT' OR TO_CHAR(Sysdate, 'DY') = 'SUN') THEN RAISE Not_on_weekends; END IF; /* check for company holidays:*/ SELECT COUNT(*) INTO Dummy FROM Company_holidays WHERE TRUNC(Day) = TRUNC(Sysdate); /* TRUNC gets rid of time parts of dates: */ IF dummy > 0 THEN RAISE Not_on_holidays; END IF; /* Check for work hours (8am to 6pm): */ IF (TO_CHAR(Sysdate, 'HH24') < 8 OR TO_CHAR(Sysdate, 'HH24') > 18) THEN RAISE Non_working_hours; END IF; EXCEPTION WHEN Not_on_weekends THEN Raise_application_error(-20324,'May not change ' ||'employee table during the weekend'); WHEN Not_on_holidays THEN Raise_application_error(-20325,'May not change ' ||'employee table during a holiday'); WHEN Non_working_hours THEN Raise_application_error(-20326,'May not change ' ||'Emp_tab table during non-working hours'); END;
See Also:
Oracle Database Security Guide for details on database security featuresTransparent Event Logging and Triggers
Triggers are very useful when you want to transparently perform a related change in the database following certain events.
The REORDER
trigger example shows a trigger that reorders parts as necessary when certain conditions are met. (In other words, a triggering statement is entered, and the PARTS_ON_HAND
value is less than the REORDER_POINT
value.)
Derived Column Values and Triggers: Example
Triggers can derive column values automatically, based upon a value provided by an INSERT
or UPDATE
statement. This type of trigger is useful to force values in specific columns that depend on the values of other columns in the same row. BEFORE
row triggers are necessary to complete this type of operation for the following reasons:
The dependent values must be derived before the INSERT
or UPDATE
occurs, so that the triggering statement can use the derived values.
The trigger must fire for each row affected by the triggering INSERT
or UPDATE
statement.
The following example illustrates how a trigger can be used to derive new column values for a table whenever a row is inserted or updated.
Note:
You may need to set up the following data structures for the example to work:ALTER TABLE Emp99 ADD( Uppername VARCHAR2(20), Soundexname VARCHAR2(20));
CREATE OR REPLACE TRIGGER Derived BEFORE INSERT OR UPDATE OF Ename ON Emp99 /* Before updating the ENAME field, derive the values for the UPPERNAME and SOUNDEXNAME fields. Users should be restricted from updating these fields directly: */ FOR EACH ROW BEGIN :new.Uppername := UPPER(:new.Ename); :new.Soundexname := SOUNDEX(:new.Ename); END;
Building Complex Updatable Views Using Triggers: Example
Views are an excellent mechanism to provide logical windows over table data. However, when the view query gets complex, the system implicitly cannot translate the DML on the view into those on the underlying tables. INSTEAD
OF
triggers help solve this problem. These triggers can be defined over views, and they fire instead of the actual DML.
Consider a library system where books are arranged under their respective titles. The library consists of a collection of book type objects. The following example explains the schema.
CREATE OR REPLACE TYPE Book_t AS OBJECT ( Booknum NUMBER, Title VARCHAR2(20), Author VARCHAR2(20), Available CHAR(1) ); CREATE OR REPLACE TYPE Book_list_t AS TABLE OF Book_t;
Assume that the following tables exist in the relational schema:
Table Book_table (Booknum, Section, Title, Author, Available)
Booknum | Section | Title | Author | Available |
---|---|---|---|---|
121001 | Classic | Iliad | Homer | Y |
121002 | Novel | Gone With the Wind | Mitchell M | N |
Library consists of library_table
(section
).
Section |
---|
Geography |
Classic |
You can define a complex view over these tables to create a logical view of the library with sections and a collection of books in each section.
CREATE OR REPLACE VIEW Library_view AS SELECT i.Section, CAST (MULTISET ( SELECT b.Booknum, b.Title, b.Author, b.Available FROM Book_table b WHERE b.Section = i.Section) AS Book_list_t) BOOKLIST FROM Library_table i;
Make this view updatable by defining an INSTEAD
OF
trigger over the view.
CREATE OR REPLACE TRIGGER Library_trigger INSTEAD OF INSERT ON Library_view FOR EACH ROW Bookvar BOOK_T; i INTEGER; BEGIN INSERT INTO Library_table VALUES (:NEW.Section); FOR i IN 1..:NEW.Booklist.COUNT LOOP Bookvar := Booklist(i); INSERT INTO book_table VALUES ( Bookvar.booknum, :NEW.Section, Bookvar.Title, Bookvar.Author, bookvar.Available); END LOOP; END; /
The library_view
is an updatable view, and any INSERTs
on the view are handled by the trigger that gets fired automatically. For example:
INSERT INTO Library_view VALUES ('History', book_list_t(book_t(121330, 'Alexander', 'Mirth', 'Y');
Similarly, you can also define triggers on the nested table booklist
to handle modification of the nested table element.
Tracking System Events Using Triggers
Fine-Grained Access Control Using Triggers: Example System triggers can be used to set application context. Application context is a relatively new feature that enhances your ability to implement fine-grained access control. Application context is a secure session cache, and it can be used to store session-specific attributes.
In the example that follows, procedure set_ctx
sets the application context based on the user profile. The trigger setexpensectx
ensures that the context is set for every user.
CONNECT secdemo/secdemo CREATE OR REPLACE CONTEXT Expenses_reporting USING Secdemo.Exprep_ctx; REM ================================================================= REM Creation of the package which implements the context: REM ================================================================= CREATE OR REPLACE PACKAGE Exprep_ctx AS PROCEDURE Set_ctx; END; SHOW ERRORS CREATE OR REPLACE PACKAGE BODY Exprep_ctx IS PROCEDURE Set_ctx IS Empnum NUMBER; Countrec NUMBER; Cc NUMBER; Role VARCHAR2(20); BEGIN -- SET emp_number: SELECT Employee_id INTO Empnum FROM Employee WHERE Last_name = SYS_CONTEXT('userenv', 'session_user'); DBMS_SESSION.SET_CONTEXT('expenses_reporting','emp_number', Empnum); -- SET ROLE: SELECT COUNT (*) INTO Countrec FROM Cost_center WHERE Manager_id=Empnum; IF (countrec > 0) THEN DBMS_SESSION.SET_CONTEXT('expenses_reporting','exp_role','MANAGER'); ELSE DBMS_SESSION.SET_CONTEXT('expenses_reporting','exp_role','EMPLOYEE'); END IF; -- SET cc_number: SELECT Cost_center_id INTO Cc FROM Employee WHERE Last_name = SYS_CONTEXT('userenv','session_user'); DBMS_SESSION.SET_CONTEXT(expenses_reporting','cc_number',Cc); END; END;
CALL Syntax
CREATE OR REPLACE TRIGGER Secdemo.Setexpseetx AFTER LOGON ON DATABASE CALL Secdemo.Exprep_etx.Set_otx
System event publication lets applications subscribe to database events, just like they subscribe to messages from other applications. The system events publication framework includes the following features:
Infrastructure for publish/subscribe, by making the database an active publisher of events.
Integration of data cartridges in the server. The system events publication can be used to notify cartridges of state changes in the server.
Integration of fine-grained access control in the server.
By creating a trigger, you can specify a procedure that runs when an event occurs. DML events are supported on tables, and system events are supported on DATABASE
and SCHEMA
. You can turn notification on and off by enabling and disabling the trigger using the ALTER TRIGGER
statement.
This feature is integrated with the Advanced Queueing engine. Publish/subscribe applications use the DBMS_AQ
.ENQUEUE
() procedure, and other applications such as cartridges use callouts.
See Also:
Oracle Streams Advanced Queuing User's Guide and Reference for details on how to subscribe to published events
When events are detected by the database, the trigger mechanism executes the action specified in the trigger. As part of this action, you can use the DBMS_AQ
package to publish the event to a queue so that subscribers receive notifications.
Note:
Only system-defined database events can be detected this way. You cannot define your own event conditions.When an event occurs, the database fires all triggers that are enabled on that event, with some exceptions:
If the trigger is actually the target of the triggering event, it is not fired. For example, a trigger for all DROP
events is not fired when it is dropped itself.
If a trigger has been modified but not committed within the same transaction as the firing event. For example, recursive DDL within a system trigger might modify a trigger, which prevents the modified trigger from being fired by events within the same transaction.
You can create more than one trigger on an object. When an event fires more than one trigger, the firing order is not defined and so you should not rely on the triggers being fired in a particular order.
When an event is published, certain runtime context and attributes, as specified in the parameter list, are passed to the callout procedure. A set of functions called event attribute functions are provided.
See Also:
"Event Attribute Functions" for information on event-specific attributesFor each supported system event, you can identify and predefine event-specific attributes for the event. You can choose the parameter list to be any of these attributes, along with other simple expressions. For callouts, these are passed as IN
arguments.
Return status from publication callout functions for all events are ignored. For example, with SHUTDOWN
events, the database cannot do anything with the return status.
See Also:
"List of Database Events" for details on return statusTraditionally, triggers execute as the definer of the trigger. The trigger action of an event is executed as the definer of the action (as the definer of the package or function in callouts, or as owner of the trigger in queues). Because the owner of the trigger must have EXECUTE
privileges on the underlying queues, packages, or procedure, this behavior is consistent.
When the database fires a trigger, you can retrieve certain attributes about the event that fired the trigger. You can retrieve each attribute with a function call. Table 9-2 describes the system-defined event attributes.
Note:
To make these attributes available, you must first run the CATPROC.SQL
script.
The trigger dictionary object maintains metadata about events that will be published and their corresponding attributes.
In earlier releases, these functions were accessed through the SYS
package. We recommend you use these public synonyms whose names begin with ora_
.
Table 9-2 System-Defined Event Attributes
This section describes important system events and client events.
System events are related to entire instances or schemas, not individual tables or rows. Triggers created on startup and shutdown events must be associated with the database instance. Triggers created on error and suspend events can be associated with either the database instance or a particular schema.
Table 9-3 contains a list of system manager events.
Table 9-3 System Manager Events
Client events are the events related to user logon/logoff, DML, and DDL operations. For example:
CREATE OR REPLACE TRIGGER On_Logon AFTER LOGON ON The_user.Schema BEGIN Do_Something; END;
The LOGON
and LOGOFF
events allow simple conditions on UID
and USER
. All other events allow simple conditions on the type and name of the object, as well as functions like UID
and USER
.
The LOGON
event starts a separate transaction and commits it after firing the triggers. All other events fire the triggers in the existing user transaction.
The LOGON
and LOGOFF
events can operate on any objects. For all other events, the corresponding trigger cannot perform any DDL operations, such as DROP
and ALTER
, on the object that caused the event to be generated.
The DDL allowed inside these triggers is altering, creating, or dropping a table, creating a trigger, and compile operations.
If an event trigger becomes the target of a DDL operation (such as CREATE TRIGGER
), it cannot be fired later during the same transaction
Table 9-4 contains a list of client events.
Table 9-4 Client Events
Event | When Fired? | Attribute Functions |
---|---|---|
BEFORE ALTER AFTER ALTER |
When a catalog object is altered. |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_dict_obj_type ora_dict_obj_name ora_dict_obj_owner ora_des_encrypted_password (for ALTER USER events) ora_is_alter_column (for ALTER TABLE events) ora_is_drop_column (for ALTER TABLE events) |
BEFORE DROP AFTER DROP |
When a catalog object is dropped. |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_dict_obj_type ora_dict_obj_name ora_dict_obj_owner |
BEFORE ANALYZE AFTER ANALYZE |
When an analyze statement is issued |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_dict_obj_name ora_dict_obj_type ora_dict_obj_owner |
BEFORE ASSOCIATE STATISTICS AFTER ASSOCIATE STATISTICS |
When an associate statistics statement is issued |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_dict_obj_name ora_dict_obj_type ora_dict_obj_owner ora_dict_obj_name_list ora_dict_obj_owner_list |
BEFORE AUDIT AFTER AUDIT BEFORE NOAUDIT AFTER NOAUDIT |
When an audit or noaudit statement is issued |
ora_sysevent ora_login_user ora_instance_num ora_database_name |
BEFORE COMMENT AFTER COMMENT |
When an object is commented |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_dict_obj_name ora_dict_obj_type ora_dict_obj_owner |
BEFORE CREATE AFTER CREATE |
When a catalog object is created. |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_dict_obj_type ora_dict_obj_name ora_dict_obj_owner ora_is_creating_nested_table (for CREATE TABLE events) |
BEFORE DDL AFTER DDL |
When most SQL DDL statements are issued. Not fired for |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_dict_obj_name ora_dict_obj_type ora_dict_obj_owner |
BEFORE DISASSOCIATE STATISTICS AFTER DISASSOCIATE STATISTICS |
When a disassociate statistics statement is issued |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_dict_obj_name ora_dict_obj_type ora_dict_obj_owner ora_dict_obj_name_list ora_dict_obj_owner_list |
BEFORE GRANT AFTER GRANT |
When a grant statement is issued |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_dict_obj_name ora_dict_obj_type ora_dict_obj_owner ora_grantee ora_with_grant_option ora_privileges |
BEFORE LOGOFF |
At the start of a user logoff |
ora_sysevent ora_login_user ora_instance_num ora_database_name |
AFTER LOGON |
After a successful logon of a user. |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_client_ip_address |
BEFORE RENAME AFTER RENAME |
When a rename statement is issued. |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_dict_obj_name ora_dict_obj_owner ora_dict_obj_type |
BEFORE REVOKE AFTER REVOKE |
When a revoke statement is issued |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_dict_obj_name ora_dict_obj_type ora_dict_obj_owner ora_revokee ora_privileges |
AFTER SUSPEND |
After a SQL statement is suspended because of an out-of-space condition. The trigger should correct the condition so the statement can be resumed. |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_server_error ora_is_servererror space_error_info |
BEFORE TRUNCATE AFTER TRUNCATE |
When an object is truncated |
ora_sysevent ora_login_user ora_instance_num ora_database_name ora_dict_obj_name ora_dict_obj_type ora_dict_obj_owner |