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Written by JODB Master
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Thursday, 15 November 2007 |
Concept and Principle
The concept of native queries is taken from the following
two papers:
- Cook/Rosenberger, Native Queries for Persistent Objects, A
Design White Paper
- Cook/Rai, Safe Query Objects: Statically Typed Objects as
Remotely Executable Queries
Native Queries provide the ability to run an arbitrary of
code against all instances of the specified class. Native query expressions
should return true to mark specific instances as part of the result set. JODB
database will optimize/transform native query expressions and run them against
indexes and without instantiating actual objects, where this is possible.
Simple Example
Predicate<Pilot> predicate= new
Predicate<Pilot>() {
public
boolean match(Pilot pilot) {
return
pilot.getPoints() > 100 && pilot.getPoints() < 500 ;
}
};
List<Pilot>
pilots = sessionContainer.query(predicate); |
The resulting set will represent all instances of Predicate object with 'points'
value greater than 100 and less than 500.
One of the obvious benefits of Native Queries is the seamless
autocompletion and code refactoring functionality available in most modern IDE's
(i.e. Eclipse).
Complex Queries and Performance
In general you can run any code as native queries but be
aware of performance penalties for complicated code. As it was mentioned before
the JODB will try to optimize queries and avoid excessive object instantiation.
For complex queries this process could be overcomplicated and force JODB to
instantiate actual instances.
To programmatically verify if an arbitrary predicate code can
be run optimized you can use the JODBSessionContainer.isOptimizedQuery(..)
method. The method especially usable if you want to safeguard your code against
none optimizable queries (i.e. by throwing an Exception).
Required 3rd Party Libraries
The process of query analysis and optimization involves ASM project hosted by ObjectWeb. In order to run native queries you'll have to add ASM libraries (included within distribution as '3rdpartylibs') to your classpath.
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