|Oracle8i Data Cartridge Developer's Guide
This chapter summarizes provides an example of starting with a schema for a data cartridge. Object types are crucial to building data cartridges in that they enable domain-level abstractions to be captured in the database.
For information about creating and using object types, following manuals also contain relevant information:
For example, the Oracle8i Application Developer's Guide - Fundamentals describes the creation of types as columns in relational tables, the use of types as rows in object tables, forward type declarations, object type dependencies, and object type references.
In the Oracle8i ORDBMS, you use object types to model real-world entities. An object type has attributes, which reflect the entity's structure, and methods, which implement the operations on the entity. Attributes are defined using built-in types or other object types. Methods are functions or procedures written in PL/SQL or an external language like C and stored in the database.
A typical use for an object type is to impose structure on some part of the data kept in the database. For example, an object type named DataStream could used by a cartridge to store large amounts of data in a character
LOBs, or large objects, are discussed elsewhere in this document and in the Oracle8i Application Developer's Guide - Large Objects (LOBs).) This object type has attributes such as an identifier, a name, a date, and so on. The following statement defines the DataStream datatype:
create or replace type DataStream as object ( id integer, name varchar2(20), createdOn date, data clob, MEMBER FUNCTION DataStreamMin return pls_integer, MEMBER FUNCTION DataStreamMax return pls_integer, MAP MEMBER FUNCTION DataStreamToInt return integer, PRAGMA restrict_references(DataStreamMin, WNDS, WNPS), PRAGMA restrict_references(DataStreamMax, WNDS, WNPS));
A method is a procedure or function that is part of the object type definition and that can operate on the object type data attributes. Such methods are called member methods, and they take the keyword
MEMBER when you specify them as a component of the object type. The DataStream type definition declares three methods. The first two, DataStreamMin and DataStreamMax, calculate the minimum and maximum values, respectively, in the data stream stored inside the character
The third method (DataStreamToInt), a map method, governs comparisons between instances of data stream type. Map methods are described in elsewhere in this document.
The pragma (compiler directive)
RESTRICT_REFERENCES is necessary for security, and is discussed below.
After declaring the type, define the type body. The body contains the code for type methods. The following example shows the type body definition for the DataStream type. It defines the member function methods (DataStreamMin and DataStreamMax) and the map method (DataStreamToInt).
create or replace type body DataStream is
MEMBER FUNCTION DataStreamMin return pls_integer is a pls_integer := DS_Package.ds_findmin(data); begin return a; end; MEMBER FUNCTION DataStreamMax return pls_integer is b pls_integer := DS_Package.ds_findmax(data); begin return b; end; MAP MEMBER FUNCTION DataStreamToInt return integer is c integer := id; begin return c; end; end;
DataStreamMin and DataStreamMax involve calling routines in a PL/SQL package called DS_Package. Since these methods are likely to be compute-intensive (they process numbers stored in the CLOB to determine minimum and maximum values), they are defined as external procedures and implemented in C. The external dispatch is routed through a PL/SQL package named DS_Package. Such packages are discussed in Oracle8i Supplied Packages Reference.
The third method (DataStreamToInt), the map method, is implemented in PL/SQL. Because we have a identifier (id) attribute in DataStream, this method can return the value of the identifier attribute. (Most map methods, however, are more complex than DataStreamToInt.).
TYPE statement has an optional keyword
OID, which associates a user-specified object identifier (
OID) with the type definition. This feature was available effective with release 8.0.3; however, it was not documented because it is intended for use primarily by Oracle product developers and by developers of data cartridges. However, it should be used by anyone who creates an object type that will be used in more than one database.
Each type has an
OID. If you create an object type and do not specify an
OID, Oracle generates an
OID and assigns it to the type. Oracle8i uses the
OID internally for operations pertaining to that type. Using the same
OID for a type is important if you plan to share instances of the type across databases for such operations as export/import and distributed queries.
For example, assume that you want to create a type named SpecialPerson and then instantiate that type in two different databases with tables named SpecialPersonTable1 and SpecialPersonTable2. The RDBMS needs to know that the SpecialPerson type is the same type in both instances, and therefore the type must be defined using the same
OID in both databases. If you do not specify an
TYPE, a unique identifier is created automatically by the RDBMS.
The syntax for specifying an
OID for an object type is as follows:
In the following example, the
SELECT statement generates an
OID, and the
TYPE statement uses the
OID in creating an object type named mytype. Be sure to use the
SELECT statement to generate a different
OID for each object type to be created, because this is the only way to guarantee that each
OID is valid and globally unique.
SVRMGR> SELECT SYS_OP_GUID() FROM DUAL; SYS_OP_GUID() -------------------------------- 19A57209ECB73F91E03400400B40BBE3 1 row selected. SVRMGR> CREATE TYPE mytype OID '19A57209ECB73F91E03400400B40BBE3' 2> AS OBJECT (attrib1 NUMBER); Statement processed.
The system implicitly defines a constructor method for each object type that you define. The name of the constructor method is the same as the name of the object type. The parameters of the constructor method are exactly the data attributes of the object type, and they occur in the same order as the attribute definition for the object type. At present, only one constructor method can be defined, and thus you cannot define other constructor methods.
For example, when the system executes the following statement to create a type named rational_type, it also implicitly creates a constructor method for this object type.
When you instantiate an object of rational_type, you invoke the constructor method. For example:
CREATE TABLE some_table ( c1 integer, c2 rational_type); INSERT INTO some_table VALUES (42, rational_type(223, 71));
SQL performs comparison operations on objects. Comparisons can be explicit, using the comparison operators (=, <, >, <>, <=, >=, !=) and the
IN predicates. Comparisons can be implicit, as in the
Comparison of objects makes use of special member functions of the object type: map methods and order methods. To perform object comparison, you must implement either a map method or order method in the
For example, the type body for the DataStream type, implements the map member function for DataStream comparison as:
This definition of the map member function relies on the presence of the id attribute of the DataStream type to map instances to integers. Whenever a comparison operation is required between objects of type DataStream, the map function DataStreamToInt () is called implicitly by the system.
The object type rational_type does not have a simple id attribute like that for DataStream. For rational_type, the map member function is slightly more complicated. Because a map function can return any of the built-in types; rational_type can return a value or type
If you have not defined a map or order function for an object type, only equality comparisons are allowed on objects of that type. Oracle SQL performs the comparison by doing a field-by-field comparison of the attributes of that type.