Triggers - polito.itmeans of triggers SQL provides instructions for defining triggers Triggers are defined by means of the DDL instruction CREATE TRIGGER Trigger syntax and semantics

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Database Systems

Triggers

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Triggers

Active Database SystemsOracle TriggersDifferences between Oracle and DB2 TriggersGuidelides in writing triggers in OracleTrigger Design

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Database Management Systems

Active Database Systems

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Active database systems

Traditional DBMS operation is passiveQueries and updates are explicitly requested by usersThe knowledge of processes operating on data is typically embedded into applications

Active database systemsReactivity is a service provided by a normal DBMSReactivity monitors specific database events and triggers actions in response

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Active database systems

Reactivity is provided by automatically executing rules Rules are in the form

EventConditionAction

Also called active or ECA rules

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Active rules

EventDatabase modification operation

ConditionPredicate on the database stateIf the condition is true, the action is executed

ActionSequence of SQL instructions or application procedure

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Rule engine

Component of the DBMS, in charge of Tracking eventsExecuting rules when appropriate

based on the execution strategy of the DBMSRule execution is interleaved with traditional transaction execution

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Example

The active rule manages reorder in an inventory stock

when the stocked quantity of a product goes below a given thresholda new order for the product should be issued

EventUpdate of the quantity on hand for product xInsert of a new product x

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Example

The active rule manages reorder in an inventory stock

when the stocked quantity of a product goes below a given thresholda new order for the product should be issued

ConditionThe quantity on hand is below a given thresholdand there are no pending orders for product x

ActionIssue an order with given reorder quantity for product x

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Applications of active rules

Internal applicationsmaintenance of complex integrity constraintsreplication managementmaterialized view maintenance

Business RulesIncorporate into the DBMS application knowledge

E.g., reorder ruleAlerters

widely used for notification

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Triggers

Commercial products implement active rules by means of triggersSQL provides instructions for defining triggers

Triggers are defined by means of the DDL instruction CREATE TRIGGER

Trigger syntax and semantics are covered in the SQL3 standard

Some commercial products implement different features with respect to the standard

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Trigger structure

EventInsert, delete, update of a tableEach trigger can only monitor events on a singletable

ConditionSQL predicate (it is optional)

ActionSequence of SQL instructionsProprietary programming language blocks

e.g. Oracle PL/SQLJava block

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Execution process

When the events take place [triggering]If the condition is true [evaluation]Then the action is executed [execution]

Seems very simple but…Execution modesExecution granularity

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Execution mode

ImmediateThe trigger is executed immediately before or afterthe triggering statement

DeferredThe trigger is executed immediately before commit

Only the immediate option is available in commercial systems

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Execution granularity

Tuple (or row level)One separate execution of the trigger for each tuple affected by the triggering statement

StatementOne single trigger execution for all tuples affected by the triggering statement

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Granularity example

Table T

Transaction statement

Trigger executionA row level trigger executes twiceA statement level trigger executes once

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A B1 52 98 20

UPDATE TSET A=A+1WHERE B<10;

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Oracle Triggers

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Trigger syntax

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CREATE TRIGGER TriggerNameMode Event {OR Event }ON TargetTable[[ REFERENCING ReferenceName]FOR EACH ROW[WHEN Predicate]]PL/SQL Block

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Trigger syntax

Mode is BEFORE or AFTERAlso INSTEAD OF but it should be avoided

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Trigger syntax

Event ON TargetTable is INSERTDELETEUPDATE [OF ColumnName]

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Trigger syntax

FOR EACH ROW specifies row level execution semantics

If omitted, the execution semantics is statement level

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Trigger syntax

The old and new states of the row triggering a row level trigger may be accessed by means of the

OLD.ColumnName variableNEW.ColumnName variable

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Trigger syntax

To rename the state variablesREFERENCING OLD AS OldVariableName

similarly for NEW

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Trigger syntax

Only for row level execution semantics (i.e., FOR EACH ROW)

A condition may be optionally specifiedThe old and new state variables may be accessed

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Trigger syntax

The action isa sequence of SQL instructionsa PL/SQL block

No transactional and DDL instructions25


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Trigger semantics

Execution modesimmediate beforeimmediate after

Granularity isrow (tuple)statement

Execution is triggered by insert, delete, or update statements in a transaction

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Execution algorithm

1. Before statement triggers are executed2. For each tuple in TargetTable affected by the

triggering statementa) Before row triggers are executedb) The triggering statement is executed

+ integrity constraints are checked on tuplesc) After row triggers are executed

3. Integrity constraints on tables are checked4. After statement triggers are executed

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Trigger semantics

The execution order for triggers with the same event, mode and granularity is not specified

it is a source of non determinismWhen an error occurs

rollback of all operations performed by the triggersrollback of the triggering statement in the triggering transaction

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Non termination

Trigger execution may activate other triggersCascaded trigger activation may lead to non termination of trigger execution

A maximum length for the cascading trigger execution may be set

default = 32 triggersIf the maximum is exceeded

an execution error is returned

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Mutating tables

A mutating table is the table modified by the statement (i.e., event) triggering the triggerThe mutating table

cannot be accessed in row level triggersmay only be accessed in statement triggers

Limited access on mutating tables only characterizes Oracle applications

accessing mutating tables is always allowed in SQL3

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Example

Trigger to manage reorder in an inventory stockwhen the stocked quantity of a product goes below a given thresholda new order for the product should be issued

The following database schema is givenInventory (Part#, QtyOnHand, ThresholdQty,

ReorderQty)PendingOrders(Part#, OrderDate, OrderedQty)

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Example


EventUpdate of the quantity on hand for product xInsert of a new product x

Execution semanticsAfter the modification eventSeparate execution for each row of the Inventory table

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Trigger example

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CREATE TRIGGER ReorderAFTER UPDATE OF QtyOnHand OR INSERT ON InventoryFOR EACH ROW

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Example


ConditionThe quantity on hand is below a given threshold

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Trigger example

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CREATE TRIGGER ReorderAFTER UPDATE OF QtyOnHand OR INSERT ON InventoryFOR EACH ROWWHEN (NEW.QtyOnHand < NEW.ThresholdQty)

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Example


ConditionThe quantity on hand is below a given thresholdand there are no pending orders for product x

This part cannot be introduced into the WHEN clause

ActionIssue an order with given reorder quantity for product x 36

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Example: Trigger body

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DECLAREN number;BEGINselect count(*) into Nfrom PendingOrderswhere Part# = :NEW.Part#;If (N=0) then

insert into PendingOrders(Part#,OrderedQty,OrderDate)values (:NEW.Part#, :NEW.ReorderQty, SYSDATE);

end if;END;

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Complete trigger example

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CREATE TRIGGER ReorderAFTER UPDATE OF QtyOnHand OR INSERT ON InventoryFOR EACH ROWWHEN (NEW.QtyOnHand < NEW. ThresholdQty)DECLAREN number;BEGINselect count(*) into Nfrom PendingOrderswhere Part# = :NEW.Part#;If (N==0) then

insert into PendingOrders(Part#,OrderedQty,OrderDate)values (:NEW.Part#, :NEW.ReorderQty, SYSDATE);

end if;END;

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Concise comparison betweenOracle and DB2 Triggers

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Differences between Oracle and DB2Oracle DB2

Reference to Old_Table and New_Table in statement triggers No Yes

When clause in statement triggers No Yes

Execution order between row and statement triggers with same mode

Specified Arbitrary

Execution order between triggers with same event, mode and granularity

Unspecified Creation Order

More than one triggering event allowed Yes No

Forbidden access to the mutating table Yes for row triggers

No

Availability of the instead semantics Yes No

Database modifications allowed in before triggers

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Guidelines in writing triggers in Oracle

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Guidelines in writing triggers in Oracle

Execution Mode INSTEAD OF is allowed in Oracle but it should be avoidedUsage of before triggers in Oracle to be compliant with the standard

Modifications of the NEW variable in tuples affected by the triggering statement are allowed in before triggersOther databases modifications apart those reported in the previous point are not allowed on before triggers Before triggers cannot trigger other triggers

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Trigger Design

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Trigger design

The design of a single trigger is usually simpleIdentify

execution semanticseventcondition (optional)action

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Trigger design

Understanding mutual interactions among triggers is more complex

The action of one trigger may be the event of a different trigger

Cascaded executionIf mutual triggering occurs

Infinite execution is possible

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Trigger execution properties

TerminationFor an arbitrary database state and user transaction, trigger execution terminates in a final state (also after an abort)

ConfluenceFor an arbitrary database state and user transaction, trigger execution terminates in a unique final state, independently of the execution order of triggers

Termination is the most important propertyConfluence is enforced by deterministic trigger execution

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Guaranteeing termination

Termination is guaranteed at run time by aborting trigger execution after a given cascading lengthTermination may be verified at design time by means of the triggering graph

a node for each triggera directed edge Ti Tj if trigger Ti is performing an action triggering trigger Tj

A cycle in the graph shows potential non terminating executions

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Example

Trigger managing salary amountsWhen a given average salary value is exceeded, a salary reduction is automatically enforced

The following table is givenEmployee (Emp#, Ename, …, Salary)

EventUpdate of the Salary attribute in EmployeeInsert into Employee

Will write only trigger for update

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Example

Trigger managing salary amountsWhen a given average salary value is exceeded, a salary reduction is automatically enforced

The following table is givenEmployee (Emp#, Ename, …, Salary)

Execution semanticsAfter the modification eventsSeparate execution for each update instruction

No condition for execution

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Example

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CREATE TRIGGER SalaryMonitorAFTER UPDATE OF Salary ON EmployeeFOR EACH STATEMENTBEGINupdate Employeeset Salary = Salary * Kwhere 2500 < (select AVG (Salary) from Employee);

END;

SalaryMonitorThe value of K may beK = 0.9 execution terminatesK = 1.1 infinite execution

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Trigger applications

Internal applicationsmaintenance of complex integrity constraintsreplication managementmaterialized view maintenance

Business RulesIncorporate into the DBMS application knowledge

E.g., reorder ruleAlerters

widely used for notification

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Triggers for constraint management

Triggers are exploited to enforce complex integrity constraintsDesign procedure1. Write the constraint as a SQL predicate

It provides a condition for the trigger execution2. Identify the events which may violate the

constrainti.e. the condition

3. Define the constraint management technique in the action

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Design example (1)

The following tables are givenSupplier S (S#, SName, …)Part P (P#, PName, …)Supply SP (S#, P#, Qty)

Constraint to be enforcedA part may be supplied by at most 10 different suppliers

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Design example (1)

Constraint predicate

set of parts violating the constraintEvents

insert on SPupdate of P# on SP

Actionreject the violating transaction

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select P#from SPgroup by P#having count(*) > 10

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Design example (1)

Execution semanticsafter the modificationstatement level

to capture the effect of the entire modification(Oracle) to allow access to the mutating table

(Oracle) No conditionThe condition cannot be specified in the WHEN clauseIt is checked in the trigger body

Design for Oracle trigger semantics

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Design example (1)CREATE TRIGGER TooManySuppliersAFTER UPDATE OF P# OR INSERT ON SPDECLAREN number;BEGINselect count(*) into Nfrom SPwhere P# IN (select P# from SP

group by P#having count(*) > 10);

if (N <> 0) thenraise_application_error (xxx, ‘constraint violated’);

end if;END;

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Design example (2)

The following tables are givenSupplier S (S#, SName, …)Part P (P#, PName, …)Supply SP (S#, P#, Qty)

Constraint to be enforcedThe quantity of a product supply cannot be larger than 1000. If it is larger, trim it to 1000.

Check constraints do not allow compensating actions

Implement with a trigger

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Design example (2)

Constraint predicateQty > 1000It is also the trigger condition

Eventsinsert on SPupdate of Qty on SP

ActionQty = 1000

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Design example (2)

Execution semanticsbefore the modification takes place

its effect can be changed before the constraint is checked

row leveleach tuple is modified separately

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Design example (2)

CREATE TRIGGER ExcessiveQtyBEFORE UPDATE OF Qty OR INSERT ON SPFOR EACH ROWWHEN (NEW.Qty > 1000)BEGIN:NEW.Qty := 1000;END;

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Triggers for materialized view maintenance

Materialized views are queries persistently stored in the database

provide increased performancecontain redundant information

e.g., aggregate computationsTriggers are exploited to maintain redundant data

Propagate data modifications on tables to materialized view

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Design example (3)

TablesStudent S (SId, SName, DCId)Degree course DC (DCId, DCName)

Materialized viewEnrolled students ES (DCId, TotalStudents)

For each degree course, TotalStudents counts the total number of enrolled studentsDefined by query

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SELECT DCId, COUNT(*)FROM SGROUP BY DCId;

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Design example (3)

TablesStudent S (SId, SName, DCId)Degree course DC (DCId, DCName)

Materialized viewEnrolled students ES (DCId, TotalStudents)

For each degree course, TotalStudents counts the total number of enrolled students

A new degree course is inserted in materialized view ES when the first student is enrolled in itA degree course is deleted from ES when the last student quits it

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Design example (3)

Database schemaS (SId, SName, DCId)DC (DCId, DCName)ES (DCId, TotalStudents)

Propagate modifications on table S to materialized view (table) ES

Inserting new tuples into SDeleting tuples from SUpdating the DCId attribute in one or more tuples of S

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Design example (3)

Design three triggers to manage separately each data modification

Insert trigger, delete trigger, update triggerAll triggers share the same execution semantics

Execution semanticsafter the modification takes place

Table ES is updated after table S has been modifiedrow level

Separate execution for each tuple of table Ssignificantly simpler to implement

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Insert trigger (3)

Eventinsert on S

No conditionIt is always executed

Actionif table ES contains the DCId in which the student is enrolled

increment TotalStudentsotherwise

add a new tuple in table ES for the degree course, with TotalStudents set to 1

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Insert trigger (3)

CREATE TRIGGER InsertNewStudentAFTER INSERT ON SFOR EACH ROWDECLAREN number;BEGIN--- check if table ES contains the tuple for the degree--- course NEW.DCId in which the student enrollsselect count(*) into Nfrom ESwhere DCId = :NEW. DCId;

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Insert trigger (3)

if (N <> 0) then--- the tuple for the NEW.DCId degree course is--- available in ESupdate ESset TotalStudents = TotalStudents +1where DCId = :NEW.DCId;

else--- no tuple for the NEW.DCId degree course is--- available in ESinsert into ES (DCId, TotalStudents)values (:NEW.DCId, 1);

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Delete trigger (3)

Eventdelete from S


Actionif the student was the only student enrolled in the degree course

delete the corresponding tuple from ESotherwise

decrement TotalStudents

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Delete trigger (3)

CREATE TRIGGER DeleteStudentAFTER DELETE ON SFOR EACH ROWDECLAREN number;BEGIN--- read the number of students enrolled on--- the degree course OLD.DCIdselect TotalStudents into Nfrom ESwhere DCId = :OLD.DCId;

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Delete trigger (3)

if (N > 1) then--- there are many enrolled studentsupdate ESset TotalStudents = TotalStudents – 1where DCId = :OLD.DCId;

else--- there is a single enrolled studentdelete from ESwhere DCId = :OLD.DCId;

end if;END;

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Update trigger (3)

EventUpdate of DCId on S


Actionupdate table ES for the degree course where the student was enrolled

decrement TotalStudents, or delete tuple if last student

update table ES for the degree course where the student is currently enrolled

increment TotalStudents, or insert new tuple if first student

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Update trigger (3)

CREATE TRIGGER UpdateDegreeCourseAFTER UPDATE OF DCId ON SFOR EACH ROWDECLAREN number;BEGIN--- read the number of students enrolled in --- degree course OLD.DCIdselect TotalStudents into Nfrom ESwhere DCId = :OLD.DCId;

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Update trigger (3)

if (N > 1) then--- there are many enrolled studentsupdate ESset TotalStudents = TotalStudents – 1where DCId = :OLD.DCId;

else--- there is a single enrolled studentdelete from ESwhere DCId = :OLD.DCId;

end if;

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Update trigger (3)

--- check if table ES contains the tuple for the degree--- course NEW.DCId in which the student is enrolledselect count(*) into Nfrom ESwhere DCId = :NEW. DCId;

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Update trigger (3)

if (N <> 0) then--- the tuple for the NEW.DCId degree course is available in ES

update ESset TotalStudents = TotalStudents +1where DCId = :NEW.DCId;

else--- no tuple for the NEW.DCId degree course is available in ES

insert into ES (DCId, TotalStudents)values (:NEW.DCId, 1);

end if;END;

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Documents

Triggers - polito.itmeans of triggers SQL provides instructions for defining triggers Triggers are defined by means of the DDL instruction CREATE TRIGGER Trigger syntax and semantics