177 UTL_REF

Using UTL_REF

• Overview

• Security Model

• Types

• Exceptions

Overview

Oracle supports user-defined composite type or object type. Any instance of an object type is called an object. An object type can be used as the type of a column or as the type of a table.

In an object table, each row of the table stores an object. You can uniquely identify an object in an object table with an object identifier.

A reference is a persistent pointer to an object, and each reference can contain an object identifier. The reference can be an attribute of an object type, or it can be stored in a column of a table. Given a reference, an object can be retrieved.

Security Model

The procedural option is needed to use this package. This package must be created under SYS (connect/as sysdba). Operations provided by this package are performed under the current calling user, not under the package owner SYS.

You can use the UTL_REF package from stored PL/SQL procedures/packages on the server, as well as from client/side PL/SQL code.

When invoked from PL/SQL procedures/packages on the server, UTL_REF verifies that the invoker has the appropriate privileges to access the object pointed to by the REF.

Thus, if UTL_REF is defined under user SYS, and user A invokes UTL_REF.SELECT to select an object from a reference, then user A (the invoker) requires the privileges to check.

When invoked from client-side PL/SQL code, UTL_REF operates with the privileges of the client session under which the PL/SQL execution is being done.

Types

An object type is a composite datatype defined by the user or supplied as a library type. You can create the object type employee_type using the following syntax:

CREATE TYPE employee_type AS OBJECT (
   name    VARCHAR2(20),
   id      NUMBER,

member function GET_ID
    (name VARCHAR2) 
   RETURN MEMBER);

The object type employee_type is a user-defined type that contains two attributes, name and id, and a member function, GET_ID().

You can create an object table using the following SQL syntax:

CREATE TABLE employee_table OF employee_type;

Exceptions

Exceptions can be returned during execution of UTL_REF functions for various reasons. For example, the following scenarios would result in exceptions:

• The object selected does not exist. This could be because either:

  1. The object has been deleted, or the given reference is dangling (invalid).

  2. The object table was dropped or does not exist.

• The object cannot be modified or locked in a serializable transaction. The object was modified by another transaction after the serializable transaction started.

• You do not have the privilege to select or modify the object. The caller of the UTL_REF subprogram must have the proper privilege on the object that is being selected or modified.

The UTL_REF package does not define any named exceptions. You may define exception handling blocks to catch specific exceptions and to handle them appropriately.

Summary of UTL_REF Subprograms

DELETE_OBJECT Procedure

This procedure deletes an object given a reference. The semantic of this subprogram is similar to the following SQL statement:

DELETE FROM object_table  
WHERE REF(t) = reference; 

Unlike the preceding SQL statement, this subprogram does not require you to specify the object table name where the object resides.

Syntax

UTL_REF.DELETE_OBJECT (
   reference IN REF "<typename>"); 

Parameters

Exceptions

May be raised.

Examples

The following example illustrates usage of the UTL_REF package to implement this scenario: if an employee of a company changes their address, their manager should be notified.

... declarations of Address_t and others...

CREATE OR REPLACE TYPE Person_t ( 
   name    VARCHAR2(64), 
   gender  CHAR(1), 
   address Address_t, 
   MEMBER PROCEDURE setAddress(addr IN Address_t) 
); 

CREATE OR REPLACE TYPE BODY Person_t ( 
   MEMBER PROCEDURE setAddress(addr IN Address_t) IS 
   BEGIN 
      address := addr; 
   END; 
); 

CREATE OR REPLACE TYPE Employee_t (  

Under Person_t: Simulate implementation of inheritance using a REF to Person_t and delegation of setAddress to it.

thePerson  REF Person_t, 
   empno      NUMBER(5), 
   deptREF    Department_t, 
   mgrREF     Employee_t, 
   reminders  StringArray_t, 
   MEMBER PROCEDURE setAddress(addr IN Address_t), 
   MEMBER procedure addReminder(reminder VARCHAR2); 
); 

CREATE TYPE BODY Employee_t ( 
   MEMBER PROCEDURE setAddress(addr IN Address_t) IS 
      myMgr Employee_t; 
      meAsPerson Person_t; 
   BEGIN 

Update the address by delegating the responsibility to thePerson. Lock the Person object from the reference, and also select it:

UTL_REF.LOCK_OBJECT(thePerson, meAsPerson); 
      meAsPerson.setAddress(addr);    

Delegate to thePerson:

UTL_REF.UPDATE_OBJECT(thePerson, meAsPerson); 
        if mgr is NOT NULL THEN 

Give the manager a reminder:

UTL_REF.LOCK_OBJECT(mgr); 
         UTL_REF.SELECT_OBJECT(mgr, myMgr); 
         myMgr.addReminder 
         ('Update address in the employee directory for' || 
         thePerson.name || ', new address: ' || addr.asString); 
         UTL_REF.UPDATE_OBJECT(mgr, myMgr); 
      END IF; 
   EXCEPTION 
      WHEN OTHERS THEN 
      errnum := SQLCODE; 
      errmsg := SUBSTR(SQLERRM, 1, 200);

LOCK_OBJECT Procedure

This procedure locks an object given a reference. In addition, this procedure lets the program select the locked object. The semantic of this subprogram is similar to the following SQL statement:

SELECT VALUE(t) 
  INTO object 
  FROM object_table t 
  WHERE REF(t) = reference 
  FOR UPDATE; 

Unlike the preceding SQL statement, this subprogram does not require you to specify the object table name where the object resides. It is not necessary to lock an object before updating/deleting it.

Syntax

UTL_REF.LOCK_OBJECT (
   reference IN REF "<typename>"); 

UTL_REF.LOCK_OBJECT (
   reference IN REF "<typename>", 
   object    IN OUT "<typename>"); 

Parameters

Exceptions

May be raised.

SELECT_OBJECT Procedure

This procedure selects an object given its reference. The selected object is retrieved from the database and its value is put into the PL/SQL variable 'object'. The semantic of this subprogram is similar to the following SQL statement:

SELECT VALUE(t) 
INTO object 
FROM object_table t 
WHERE REF(t) = reference; 

Unlike the preceding SQL statement, this subprogram does not require you to specify the object table name where the object resides.

Syntax

UTL_REF.SELECT_OBJECT (
   reference IN REF "<typename>", 
   object    IN OUT "<typename>"); 

Parameters

Exceptions

May be raised.

UPDATE_OBJECT Procedure

This procedure updates an object given a reference. The referenced object is updated with the value contained in the PL/SQL variable 'object'. The semantic of this subprogram is similar to the following SQL statement:

UPDATE object_table t 
SET VALUE(t) = object 
WHERE REF(t) = reference; 

Unlike the preceding SQL statement, this subprogram does not require you to specify the object table name where the object resides.

Syntax

UTL_REF.UPDATE_OBJECT (
   reference IN REF "<typename>", 
   object    IN     "<typename>"); 

Parameters

Exceptions

May be raised.