When several persistent classes are in superclass/subclass hierarchy, there are two ways in which InterSystems IRIS can store their data. The default scenario is by far the most common.
Default SQL Projection of Subclasses
The class compiler projects a flattened representation of a persistent class, such that the projected table contains all the appropriate fields for the class, including those that are inherited. Hence, for a subclass, the SQL projection is a table composed of:
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All the columns in the extent of the superclass
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Additional columns based on properties only in the subclass
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Rows that represent the saved instances of the subclass
Furthermore, in the default scenario, the extent of the superclass contains one record for each saved object of the superclass and all its subclasses. The extent of each subclass is a subset of the extent of the superclass.
For example, consider the persistent classes Sample.Person and Sample.Employee in SAMPLES. The Sample.Employee class inherits from Sample.Person and adds some additional properties. In the SAMPLES, both classes have saved data.
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The SQL projection of Sample.Person is a table that contains all the suitable properties of the Sample.Person class. The Sample.Person table contains one record for each saved instance of the Sample.Person class and each saved instance of the Sample.Employee class.
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The Sample.Employee table includes the same columns as Sample.Person and also includes columns that are specific to the Sample.Employee class. The Sample.Employee table contains one record for each saved instance of the Sample.Employee class.
To see this, use the following SQL queries. The first lists all instances of Sample.Person and shows their properties:
SELECT * FROM Sample.Person
The second query lists all instances of Sample.Employee and their properties:
SELECT * FROM Sample.Employee
Notice that the Sample.Person table contains records with IDs in the range 1 to 200. The records with IDs in the range 101 to 200 are employees, and the Sample.Employee table shows the same employees (with the same IDs and with additional columns). The Sample.Person table is arranged in two apparent groups only because of the artificial way that the SAMPLES database is built. The Sample.Person table is populated and then the Sample.Employee table is populated.
Typically, the table of a subclass has more columns and fewer rows than its parent. There are more columns in the subclass because it usually adds additional properties when it extends the parent class; there are often fewer rows because there are often fewer instances of the subclass than the parent.
Alternative SQL Projection of Subclasses
The default projection is the most convenient, but on occasion, you might find it necessary to use the alternative SQL projection. In this scenario, each class has its own extent. To cause this form of projection, include the following in the definition of the superclass:
[ NoExtent ]
For example:
Class MyApp.MyNoExtentClass [ NoExtent ]
{
//class implementation
}
Each subclass of this class then receives its own extent.
If you create classes in this way and use them as properties of other classes, see Variation: CLASSNAME Parameter.
