Read Trigger
Read triggers have been implemented in OS/400 to satisfy a U.S. federal statute (HIPAA) requiring that all access to patient records in the medical profession be logged and auditable.
Read triggers are supported only via external triggers and are implemented via the new read trigger event, and there is no support for a read trigger with SQL triggers. The reason for this is that a read trigger has a very specific function; it is not intended for general use because indiscriminate use can have a significant negative impact on system performance and throughput due to System i and iSeries performance characteristics being altered.
Any read-only access to a file with a read trigger defined—either user program or system function—will cause the trigger to fire. This means that using the Copy File (CPYF) command, the Display Physical File Member (DSPPFM) command, query products like Query/400 or SQL, or user-written display or report programs will cause a read trigger to fire every time a record is accessed in each of the above scenarios. If you access one million records, you just fired the read trigger one million times. Get the picture?
The use of a read trigger will disable some system functions that are used to optimize system performance and throughput. For example, a read trigger prevents adaptive blocking and double buffering when processing a file sequentially (arrival sequence) and asynchronous fetch of the next record in keyed sequence when processing a file sequentially by key or index. This means that records are read from the file one at time instead of in optimized blocks, dramatically increasing file I/O time.
The effect is that batch programs using these processing techniques on a file with a read trigger defined will run orders of magnitude longer. This negative impact on performance can be mitigated to a certain extent by increasing CPU power, adding memory, and adding disk arms.
Multiple Triggers per Database or Trigger Event
OS/400 releases prior to V5R1 limited a file to a maximum of six triggers per file. Many software providers now include triggers in their application packages, which can conflict with the trigger requirements of System i and iSeries users.
When conflicting trigger requirements occur, it can be very difficult or impossible to combine two or more trigger programs into one. IBM's V5R1 solution was to increase the limit of six triggers to 300 and to allow more than one trigger with the same trigger time and trigger event to be defined for a file.
When multiple triggers are defined for the same database event, the triggers are fired in the order they were created. The first trigger created is the first one fired; the last created is the last one fired.
This raises some interesting questions when a trigger has to be deleted and recreated. Suppose the first trigger on the execution list must be deleted and then recreated. It then moves from being first on the execution list to last on the list. The question that you must ask is if the trigger depends on its place in the execution list to function properly. If the answer to that question is "yes, the trigger firing order must be maintained," you will need to create two CL or SQL scripts: One script removes all the triggers for the file in question, and the second recreates all the triggers in the required sequence.
Named Triggers
The increase to 300 triggers per file means that the combination of trigger time and trigger event can no longer be used to identify a trigger. Starting in V5R1, both external and SQL triggers are given a name to provide unique identification when they are created. The trigger name must be unique within a given library (not per file) and can be a maximum of 128 characters long. If you do not provide a name when the trigger is created, DB2 UDB will create a default name, and I guarantee you will not like it!
To support the naming of a trigger, a trigger name and library parameters have been added to the ADDPFTRG and RMVPFTRG commands and the new CREATE TRIGGER and DROP TRIGGER SQL statements (more on these SQL statements later).
Change Physical File Trigger (CHGPFTRG) Command
The CHGPFTRG command changes the state of one or all triggers defined for a file or table. There are two possible trigger states: disabled or enabled. When a trigger is disabled, its corresponding trigger program will not be invoked when the trigger event is satisfied. When a trigger is enabled, its corresponding trigger program will be invoked when the trigger event is satisfied.
This eliminates the problem of having to delete a trigger to disable it and then recreate the trigger again to enable it in prior releases of OS/400. Following is an example of the CHGPFTRG command syntax.
TRGLIB(Lib_Name) STATE(*DISABLED)
System Catalog Trigger Enhancements
As an aid in managing the V5R1 trigger enhancements, IBM has added four new files to the System Catalog to log and store information about triggers:
- SYSTRIGGERS contains one row for each trigger in a library for both external and SQL triggers.
- SYSTRIGCOL contains one row for each column or field either implicitly or explicitly referenced in the WHEN clause or the SQL statements for an SQL trigger.
- SYSTRIGDEP contains one row for each object referenced in the WHEN clause or SQL statements for an SQL trigger.
- SYSTRIGUPD contains one row for each column identified in the UPDATE column list, if any.
SQL Triggers
SQL triggers are a V5R1 enhancement and use one or more SQL statements (instead of a user-provided or written program) within the trigger body to perform the desired action when the trigger fires. SQL trigger support in V5R1 is a superset of the support found in DB2 UDB Version 7.1 and provides an industry-standard method for defining and managing triggers that has a high degree of portability to other database management systems.
SQL triggers use IBM's SQL procedural language to implement the trigger implementation, and they also provide more granularity and function than external triggers with column or field-level triggers, row or record-level triggers, and (SQL) statement-level triggers.
An SQL trigger can be added to a file or table with the CREATE TRIGGER statement and can be removed with the DROP TRIGGER statement, or both can be done with the Database function in iSeries Navigator.
SQL Trigger Components
In V5R1, an external trigger has the following five components (remember that the trigger name was just added in V5R1): base file or table, trigger name, trigger event, trigger time, and trigger program.
An SQL trigger has the same first four components; however, the trigger program is replaced by five additional components: trigger granularity, transition variables, transition tables, trigger mode, and triggered action.
The base file or table is the physical file or table to which the trigger is added. The trigger name provides unique trigger identification within a library. The trigger event is the condition that causes the trigger to fire. It can be the insert of a new record or row, the delete of an existing row, the update of an existing row or column, or, in very limited circumstances, the read of an existing row (see "read trigger" earlier in this article). The trigger time is when the triggered action will be performed, either before or after the trigger event completes. The trigger granularity, in conjunction with the trigger event, determines what causes the trigger to fire. Granularity can be at the column or field level, row or record level, or (SQL) statement level.
Column-level triggers are an extension of the Update trigger event and are available only with SQL triggers. Only an update of those columns listed as part of the update trigger event will cause the trigger to fire and the triggered action to be performed.
The following SQL syntax shows how the column names are listed:
If no columns are listed in the UPDATE OF clause, then an update to any column defined in the row causes the associated trigger to fire.
With a row-level trigger, the associated trigger is fired and the triggered action is performed each time the trigger event is satisfied. If the trigger condition is never satisfied, the triggered action is never performed. An SQL trigger is defined as a row-level trigger with the FOR EACH ROW clause. An external trigger is implicitly a row-level trigger.
Statement-level triggers are available only with SQL triggers, and the triggered action is performed only once per trigger event, regardless of the number of rows processed. If the trigger event is never satisfied, the triggered action is still performed once at the end of the SQL statement processing. A statement-level trigger can be used only in conjunction with a trigger time of After and a trigger mode of DB2SQL (more on trigger mode later) and is defined with the FOR EACH STATEMENT clause.
Transition variables provide the same function as the before and after images in the trigger buffer used with external triggers. They provide qualification of the column names for the image of the single row that caused the trigger to fire, before and/or after the trigger event has completed.
Transition variables are not valid with statement-level triggers. They are defined with the OLD ROW clause for the before image and the NEW ROW clause for the after image.
The following SQL syntax is used to describe and reference transition variables as part of an SQL trigger.
...
...WHERE Newrow.Salary > Oldrow.salary + 10000...
A transition table provides a function analogous to the before and after images in the trigger buffer used with external triggers and is a temporary table that contains the image of all rows affected before and/or after the trigger event completes. Since a single SQL statement can process multiple rows in a file, a mechanism is needed to be able to track/log the activity on those rows processed. Transition tables provide that capability.
A transition table can be used only in conjunction with a trigger time of After and a trigger mode of DB2SQL. It is defined with the OLD TABLE clause for a before image of all affected rows and the NEW TABLE clause for an after image of all affected rows.
The following SQL syntax is used to describe and reference a transition table as part of an SQL trigger.
...
...(SELECT COUNT(*) FROM Old_Table_Name)...
There are two trigger modes: DB2ROW and DB2SQL. A mode of DB2ROW causes the trigger to fire after each row operation and is valid only with row-level triggers. It is an exclusive function of DB2 UDB for System i and iSeries and is not available in other DB2 UDB implementations. A mode of DB2SQL causes the trigger to fire after all row operations are complete and is valid only with a trigger time of After. If it is specified for a row-level trigger, the triggered action is executed n times after all row operations, where n equals the number of rows processed. This is not as efficient as DB2ROW, since each row is effectively processed twice.
The triggered action is analogous to the trigger program in external triggers and has three parts: the SET OPTION clause, the WHEN clause, and the SQL trigger body:
The SET OPTION clause specifies the options that will be used to create the trigger.
The WHEN clause specifies the search or selection criteria or the execution criteria for the trigger body. In other words, it specifies when the SQL statements in the trigger body will be executed.
The SQL trigger body contains one or more SQL statements that perform the desired action when the trigger fires. Multiple SQL statements in the trigger body are delineated with the BEGIN and END statements. Each complete SQL statement in the trigger body must be ended with a semicolon (;).
The standard DDL and DML SQL statements—such as SELECT, INSERT, DELETE, and CREATE—can be used in the trigger body along with IBM's SQL procedural language.
An SQL trigger can be added to a file with the CREATE TRIGGER statement and removed with the DROP TRIGGER statement. The SQL syntax for these statements is shown below.
Detailed SQL Syntax for Create Trigger Statement
>>--CREATE TRIGGER--trigger-name--+-+-------------+-BEFORE-+-->
+-AFTER------------------+
>--+--INSERT--------------------------+--ON--table-name------->
|--DELETE--------------------------|
+--UPDATE--+---------------------+-+
| +-,-----<-----+ |
| | | |
+-OF-+-column-name-+--+
>-+---------------------------------------------------------+->
| +-----------------------<-----------------+ |
| | | |
| | +-ROW-+ +-AS-+ | |
+-REFERENCING-+-+-OLD-+-----+-+----+-correlation-name-+-+-+
| |
| +-ROW-+ +-AS-+ |
+-NEW-+-----+-+----+-correlation-name-+
| |
| +-AS-+ |
+-OLD TABLE-+----+---table-identifier-+
| |
| +-AS-+ |
+-NEW TABLE-+----+---table-identifier-+
+--FOR EACH STATEMENT--+ +--MODE DB2SQL--+
| | | |
>--+----------------------+--+---------------+---------------->
| | | |
+--FOR EACH ROW--------+ +--MODE DB2ROW--+
>--+-----------------------------------+---------------------->
| |
+--SET OPTION---option-statement----+
>--+---------------------------------------------------+------>
| |
+--WHEN--(--trigger-body-execution-criteria------)--+
>--SQL-trigger-body------------------------------------------><
Detailed SQL Syntax for Drop Trigger Statement
When SQL triggers are created, they have an implicit attribute of ALWREPCHG(*YES). This attribute must be explicitly specified when using external triggers; otherwise, it defaults to ALWREPCHG(*NO).
SQL Trigger Examples
Column-Level Trigger with Simple Trigger Body
BEFORE UPDATE OF salary ON emp
REFERENCING NEW AS new OLD AS old
FOR EACH ROW MODE DB2ROW
WHEN (new.salary > 1.5 * old.salary)
SET new.salary = 1.5 * old.salary;
The SQL trigger created in this example is called empsal and is fired before the update of a row in the table or file called salary. Transition variables called new and old have been defined for new row (the after image) and old row (the before image) and will be used to qualify field names referenced in the trigger body. The trigger is a row-level trigger, and its mode is DB2ROW.
The SQL statement in the trigger body will be executed when new.salary equals 1.5 times the old.salary. When this criteria is satisfied, the new.salary is set to 1.5 times the old salary. Note that this is the preferred method for modifying or changing data before it is actually written to a table or file.
Row-Level Trigger with Complex Trigger Body
AFTER INSERT ON expenses
REFERENCING NEW ROW AS n
FOR EACH ROW
MODE DB2ROW
WHEN (n.totalamount > 10000)
BEGIN
DECLARE emplname CHAR(30);
SET emplname =
(SELECT lname FROM employee
WHERE empid = n.empno);
INSERT INTO travel_audit
VALUES(n.empno, emplname, n.deptno,
n.totalamount, n.enddate);
END
The SQL trigger created in this example is called big_spenders and is fired after the insert of a row in the table or file called expenses. A transition variable called n has been defined for new row (the after image) and will be used to qualify field names referenced in the trigger body. The trigger is a row-level trigger, and its mode is DB2ROW.
Note that there are multiple SQL statements in the trigger body and that they are delineated with a BEGIN and END statement. Also note that each SQL statement is ended with a semicolon. These SQL statements in the trigger body will be executed when n.totalamount is greater than 10,000. When this criteria is satisfied, the SLQ statements in the trigger body are executed, and the result is that a row is inserted into the table called travel_audit, which contains the columns or fields called n.empno, emplname, n.deptno, n.totalamount, and n.enddate.
New Trigger Enhancements
IBM has provided significant trigger enhancements in V5R1.The maximum number of triggers per file has been raised from 6 to 300. To support 300 triggers per file, triggers are now given a name, which must be unique within a library, when they are created. The new CHGPFTRG command allows you to easily disable/enable a single trigger or a group of related triggers, and the System Catalog has had four new files added to it to log and store trigger information.
SQL triggers provide a new level of function and use one or more SQL statements (instead of a user-provided/written program) within the trigger body to perform the desired action when the trigger fires. SQL trigger support in V5R1 provides an industry-standard method for defining and managing triggers that has a high degree of portability to other database management systems. SQL triggers also provide more granularity and function than external triggers, with column or field-level triggers, row or record-level triggers, and statement-level (SQL) triggers.
Lastly, there is the new read trigger, which is not intended for general use, so you must approach it with extreme caution. Use of read triggers can have a severe detrimental impact on System i and iSeries performance and throughput.
The V5R1 trigger enhancements offer additional options for fulfilling application requirements. With proper use, these enhancements can be very beneficial to programmers.
Skip Marchesani retired from IBM after 30 years and is now a consultant with Custom Systems Corp. and one of the founding partners of System i Developer. Skip spent much of his IBM career working with the Rochester Development Lab on projects for S/38 and AS/400 and was involved with the development of the AS/400. He was part of the team that taught early AS/400 education to customers and IBM lab sites worldwide.
Skip is recognized as an industry expert on DB2 UDB (aka DB2/400) and author of the book DB2/400: The New AS/400 Database. He specializes in providing customized education for any area of the System i, iSeries, and AS/400; does database design and design reviews; and performs general System i, iSeries, and AS/400 consulting for interested clients. He has been a speaker for user groups, technical conferences, and System i, iSeries, and AS/400 audiences around the world. He is an award-winning COMMON speaker and has received its Distinguished Service Award.
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