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Active database concepts

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1 Active database concepts

2 Introduction DB triggered action triggering event think of examples

3 Rules specify actions that are automatically triggered by certain events generalised model: EVENT-CONDITION-ACTION (ECA) event(s): database update operation(s) condition: determines whether the rule is to be executed after the event occurred action: action to be taken SQL command(s) external program rules are sometimes called triggers however, some systems make distinctions between them (Postgres)

4 Example - pseudocode consider the following relations:
Employee ( Id, Name, Address, …, Salary, Dept, …) Department (Dept, Name, …, Tot_Sal, …) “Tot_Sal” represents the salary of all the employees in the department

5 Insert new employee CREATE RULE Totsal1 AFTER INSERT ON Employee
FOR EACH ROW WHEN ( NEW.Dept IS NOT NULL) UPDATE Department SET Tot_Sal = Tot_Sal + NEW.Salary WHERE Dept = NEW.Dept ;

6 Activity design similar rules for the following cases:
updating the salary of certain employees deleting certain employees

7 Observation the situation in the previous example should not occur in an “industrial” database the total salary per department can be computed from the individual salary of each employee, therefore should not be explicitly represented the example was used for didactic reasons however, could you think of real life situations where such a design would be useful?

8 Example - more realistic
Employee ( Id, Name, …, Salary, Manager, …) (1)if the salary of the manager is increased then increase the salary of all the employees that are supervised by this manager with 10% of the manager’s increase (2)if the salary of an employee is increased by more than 10% inform the general manager

9 Increase salary CREATE RULE IncreaseSal
AFTER UPDATE OF Salary ON Employee FOR EACH ROW UPDATE Employee SET Salary = Salary + (NEW.Salary - OLD.Salary) * 0.1 WHERE Manager = NEW.Id ;

10 Inform general manager
CREATE RULE InformSal AFTER UPDATE OF Salary ON Employee FOR EACH ROW WHEN ( NEW.Salary - OLD.Salary) / OLD.Salary > 0.1 Inform_general_manager(NEW.Name, NEW.Salary); --the action is an external procedure

11 Issues about active databases
general model (remember): ECA event in general INSERT, UPDATE, DELETE can be more specific (see “update of salary on employee”) condition could be empty when solely the event triggers row-level rule: the rule is triggered for each tuple for which the condition is satisfied statement-level rule: the rule is triggered only once for each triggering statement action SQL statement or external procedure

12 Row level and statement-level rule
Safe_box(ID, Name, …, Value, …) statement level inform the manager whenever items are removed from the safe tuple level inform the manager for every item more expensive than £1000 removed from the safe

13 Statement vs row level rules
--statement level CREATE RULE InformManager BEFORE DELETE ON Safe-box Inform_general_manager(); --row level FOR EACH ROW WHEN OLD.Value > 1000 Inform_general_manager(OLD.Id, OLD.Value);

14 Further issues about active databases
activate / deactivate rules + sets of rules allows for better customisation rule consideration - condition evaluation see diagram next slide immediate: evaluated as part of the triggering transaction (before, after or instead of executing the triggering event) deferred: evaluated at the end of the triggering transaction detached: evaluated as a separate transaction executing the rule action immediate, deferred or detached problems determining consistency and termination

15 Condition evaluation and rule execution
Rule R action condition triggering event INSERT INTO rel1 ... transaction T DELETE … UPDATE … INSERT INTO rel1 VALUES (…); SELECT … SELECT ... DB

16 Rules and SQL SQL2 does not implement rules SQL3 will implement rules
foreign key rules can be regarded as elements of active databases SQL3 will implement rules syntax very similar to the pseudocode used so far (which, in turn, is very similar to the Oracle syntax)

17 Active databases in Postgres
rules allow the definition of extra or alternate actions on updates triggers allow the association of user supplied procedures (functions) with database events

18 Rules in Postgres CREATE RULE <name> AS
ON <event> TO <object> [ WHERE <condition> ] DO [ INSTEAD ] [ <action> | NOTHING ] <event> ::= SELECT | UPDATE | INSERT | DELETE <object> ::= <table> | <table>.<column> <condition> ::= <SQL statement> /* “new” and “old” */ <action> ::= <SQL statement> /* “new” and “old” */

19 Foreign key integrity with rules in Postgres
CREATE RULE FK_integrity_delete AS ON delete TO items WHERE exists (select * from items_order where items_order.items_id = old.items_id) DO INSTEAD NOTHING; the rules mechanism is used to implement the view system for further details refer to the lab handouts

20 Triggers in Postgres CREATE TRIGGER <name>
{ BEFORE | AFTER } { <event> [OR <event> ...] } ON <table> FOR EACH { ROW | STATEMENT } EXECUTE PROCEDURE <function> ( <arguments> ) <event> ::= UPDATE | INSERT | DELETE <function> ::= <user supplied function> statement triggers are not (yet) implemented

21 Applications for active databases
notification of the occurrence of certain conditions in the database enforcement of integrity constraints for business rules automatic maintenance of derived data e.g. snapshots (materialised views), replicated tables, ...

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