Clusters and Other Data Structures for Oracle - Extended Logic for Data
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There are a several features that have been added to the Oracle database that are not unique data structures, but rather shape the way you can use the data in the database: the Rules Manager and the Expression Filter.
Rules Manager
The database has been continually extending the functionality it can provide, from mere data storage, which still enforced some logical attributes on data, to stored procedures. The Rules Manager, introduced with Oracle Database 10g Release 2, takes this extension a step further.
The concept behind the Rules Manager is simple. A rule is stored in the database and is called and evaluated by applications. If business conditions or requirements change, the rule covering those scenarios can be changed without having to touch the application code. Rules can be shared across multiple application systems, bringing standardization along with reduced maintenance across the set of applications. You can also create granular rules that can be used in different combinations to implement a variety of conditions.
Rules are invoked by events. The event causes the rule to be evaluated and results in a rule action being performed, either immediately or at some later time.
The Rules Manager follows the event-condition action structure and helps users to define five elements required for a Rules Manager application:
Define an event structure, which is an object in your Oracle database. Different events have different values for the attributes of the event object.
Create rules, which include conditions and their subsequent actions.
Create rule classes to store and group rules with similar structures.
Create PL/SQL procedures to implement rules.
Define a results view to configure the rules for external use when the PL/SQL actions cannot be called, such as an application that runs on multiple tiers and has rule actions that are invoked from the application server tier.
You can define conflict resolution routines to handle situations where more than one rule is matched by an event. The Rules Manager also can aggregate different events into composite events and maintain state information until all events are received.
Using rules can be a very powerful tool for implementing complex logic, but the use of rules can affect your application design. For more information on the Rules Manager, please refer to the Oracle documentation.
The Expression Filter
The Expression Filter, available since Oracle Database 10g, uses the Rules Manager to work with expressions. An expression is another object type that contains attributes evaluated by the Expression Filter. You add a VARCHAR2 column to a table that stores the values for the attributes of an expression, use a PL/SQL built-in package to add the expression to the column, and use standard SQL to set the values for the expression. To compare values to an expression, you use the EVALUATE operator in the WHERE clause of your SQL statement.
Expressions can be used to define complex qualities for rows, since an expression can have many attributes. You can also use expressions to implement many-to-many relationships without an intermediary table by using expressions from two tables to join the tables.
With the Enterprise Edition of Oracle, you can add an index to an expression, which can provide the same performance benefits of an index to the qualities defined as an expression.
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 This article is excerpted from chapter four of the book Oracle Essentials, Fourth Edition Oracle Database 11g, written by Rick Greenwald, Robert Stackowiak, and Jonathan Stern (O'Reilly, 2007; ISBN: 0596514549). Check it out today at your favorite bookstore. Buy this book now.
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