3 Defining Object Types

Objects and Object Types

In the Oracle Object-Relational Database Management System (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 in the database. For example, an object type named DataStream could be used by a cartridge to store large amounts of data in a character LOB (a data type for large objects). This object type has attributes such as an identifier, a name, a date, and so on. The statement in Example 3-1 defines the DataStream data type:

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 LOB.

The third method, DataStreamToInt, a map method, governs comparisons between instances of data stream type.

The pragma (compiler directive) RESTRICT_REFERENCES is necessary for security, and is discussed in the following sections.

After declaring the type, define the type body. The body contains the code for type methods. Example 3-2 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 are call routines in a PL/SQL package named 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 Oracle Database PL/SQL Packages and Types Reference.

The third method, DataStreamToInt, 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.

Assigning an Object Identifier to an Object Type

The CREATE TYPE statement has an optional keyword OID, which associates a user-specified object identifier (OID) with the type definition. It necessary to anyone who creates an object type used in several database.s

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. Oracle 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.

Consider creating a SpecialPerson type, and then instantiating this type in two different databases in tables named SpecialPersonTable1 and SpecialPersonTable2. The RDBMS must 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 OID with CREATE TYPE, a unique identifier is created automatically by the RDBMS. The syntax for specifying an OID for an object type is in Example 3-3.

CREATE OR REPLACE TYPE type_name OID 'oid' AS OBJECT (attribute datatype [,...]);

In Example 3-4, the SELECT statement generates an OID, and the CREATE 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.

SQLPLUS> SELECT SYS_OP_GUID() FROM DUAL; 
SYS_OP_GUID()                    
-------------------------------- 
19A57209ECB73F91E03400400B40BBE3 
1 row selected. 
 
SQLPLUS> CREATE TYPE mytype OID '19A57209ECB73F91E03400400B40BBE3'
     2> AS OBJECT (attrib1 NUMBER); 
Statement processed.

Constructor Methods

Oracle implicitly defines a constructor method for each object type that you define. The name of the constructor method is identical to 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. Only one constructor method can be defined for each object type.

In Example 3-5, the system creates a type named rational_type and implicitly creates a constructor method for this object type.

CREATE TYPE rational_type (
     numerator integer,
     denominator integer);

When you instantiate an object of rational_type, you invoke the constructor method, as demonstrated in Example 3-6:

CREATE TABLE some_table (
     c1 integer, c2 rational_type);
INSERT INTO some_table
     VALUES (42, rational_type(223, 71));

Object Comparison

SQL performs comparison operations on objects. Some comparisons are explicit, using the comparison operators (=, <, >, <>, <=, >=, !=) and the BETWEEN and IN predicates. Other comparisons are implicit, as in the GROUP BY, ORDER BY, DISTINCT, and UNIQUE clauses.

Comparison of objects uses special member functions of the object type: map methods and order methods. To perform object comparison, you must implement either a map method or an order method in the CREATE TYPE and CREATE TYPE BODY statements. In Example 3-7, the type body for the DataStream type implements the map member function:

MAP MEMBER FUNCTION DataStreamToInt return integer is 
      c integer := id; 
      begin return c; end; 

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 DataStream. Instead, its map member function is complicated, as demonstrated in Example 3-8. Because a map function can return any of the built-in types, rational_type can return a value or type REAL.

MAP MEMBER FUNCTION RationalToReal RETURN REAL IS
     BEGIN
         RETURN numerator/denominator;
     END;
...

If you have not defined a map or order function for an object type, it can only support equality comparisons. Oracle SQL performs the comparison by doing a field-by-field comparison of the attributes of that type.