5
Methods: Using PL/SQL

This chapter describes how to use PL/SQL to implement the methods of a data cartridge. Methods are procedures and functions that define the operations permitted on data defined using the data cartridge. Topics include:

Methods

A method is procedure or function that is part of the object type definition, and that can operate on the attributes of the type. Such methods are also called member methods, and they take the keyword MEMBER when you specify them as a component of the object type.

See the Oracle9i Database Concepts manual for information about:

Method specification
Method names
Method name overloading

Map methods, which govern comparisons between object types, are discussed above.

The following sections show simple examples of implementing a method, invoking a method, and referencing an attribute in a method. For further explanation and more detailed examples, see the chapter on object types in the PL/SQL User's Guide and Reference.

Implementing Methods

To implement a method, create the PL/SQL code and specify it within a CREATE TYPE BODY statement.

For example, consider the following definition of an object type named rational_type:

CREATE TYPE rational_type AS OBJECT
( numerator INTEGER,
  denominator INTEGER,
  MAP MEMBER FUNCTION rat_to_real RETURN REAL,
  MEMBER PROCEDURE normalize,
  MEMBER FUNCTION plus (x rational_type)
       RETURN rational_type);

The following definition is shown merely because it defines the function gcd, which is used in the definition of the normalize method in the CREATE TYPE BODY statement later in this section.

CREATE FUNCTION gcd (x INTEGER, y INTEGER) RETURN INTEGER AS
-- Find greatest common divisor of x and y. For example, if
-- (8,12) is input, the greatest common divisor is 4.
-- This will be used in normalizing (simplifying) fractions.
-- (You need not try to understand how this code works, unless
--  you are a math wizard. It does.)
--
   ans INTEGER;
BEGIN
   IF (y <= x) AND (x MOD y = 0) THEN
      ans := y;
   ELSIF x < y THEN 
      ans := gcd(y, x);  -- Recursive call
   ELSE
      ans := gcd(y, x MOD y);  -- Recursive call
   END IF;
   RETURN ans;
END;

The following statement implements the methods (rat_to_real, normalize, and plus) for the object type rational_type:

CREATE TYPE BODY rational_type
( MAP MEMBER FUNCTION rat_to_real RETURN REAL IS
   -- The rat-to-real function converts a rational number to 
   -- a real number. For example, 6/8 = 0.75
   BEGIN
      RETURN numerator/denominator;
   END;

   -- The normalize procedure simplifies a fraction.
   -- For example, 6/8 = 3/4
   MEMBER PROCEDURE normalize IS
      divisor INTEGER := gcd(numerator, denominator);
   BEGIN
      numerator := numerator/divisor;
      denominator := denominator/divisor;
   END;

   -- The plus function adds a specified value to the
   -- current value and returns a normalized result.
   -- For example, 1/2 + 3/4 = 5/4
   -- 
   MEMBER FUNCTION plus(x rational_type)
            RETURN rational_type IS
            -- Return sum of SELF + x
   BEGIN
      r = rational_type(numerator*x.demonimator +
             x.numerator*denominator,
             denominator*x.denominator);
                 -- Example adding 1/2 to 3/4:
                 -- (3*2 + 1*4) / (4*2)
      -- Now normalize (simplify). Here, 10/8 = 5/4
      r.normalize;
      RETURN r;
   END;
END;

Note:

If an object type has no methods, no CREATE TYPE BODY statement for that object type is required.

Invoking Methods

To invoke a method, use the following syntax:

<object_name>.<method_name>([parameter_list])

In SQL statements only, you can use the following syntax:

<correlation_variable>.<method_name>([parameter_list])

The following PL/SQL example invokes a method named get_emp_sal:

DECLARE
   employee employee_type;
   salary number;
   ...
BEGIN
   salary := employee.get_emp_sal();
   ...
END;

An alternative way to invoke a method is by using the SELF built-in parameter. Because the implicit first parameter of each method is the name of the object on whose behalf the method is invoked, the following example performs the same action as the line after BEGIN in the preceding example:

salary := get_emp_sal(SELF => employee);

In this example, employee is the name of the object on whose behalf the get_emp_sal method is invoked.

Referencing Attributes in a Method

As shown in the example in "Implementing Methods", member methods can reference the attributes and member methods of the same object type without using a qualifier. A built-in reference is always provided to the object on whose behalf the method is invoked. This reference is called SELF.

Consider the following trivial example, in which two statements set the value of variable var1 to 42:

CREATE TYPE a_type AS OBJECT (
   var1 INTEGER,
   MEMBER PROCEDURE set_var1);
CREATE TYPE BODY a_type (
   MEMBER PROCEDURE set_var1 IS
   BEGIN
      var1 := 42;
      SELF.var1 := 42;
   END set_var1;
);

In this example, var1 := 42 and SELF.var1 := 42 are in effect the same statement. Because var1 is the name of an attribute of the object type a_type and because set_var1 is a member method of this object type, no qualification is required to access var1 in the method code. However, for code readability and maintainability, you can use the keyword SELF in this context to make the reference to var1 more clear.

PL/SQL Packages

A package is a group of PL/SQL types, objects, and stored procedures and functions. The specification part of a package declares the public types, variables, constants, and subprograms that are visible outside the immediate scope of the package. The body of a package defines the objects declared in the specification, as well as private objects that are not visible to applications outside the package.

The following example shows the package specification for the package named DS_package. This package contains the two stored functions ds_findmin and ds_findmax, which implement the DataStreamMin and DataStreamMax functions defined for the DataStream object type.

create or replace package DS_package as 
    function  ds_findmin(data clob) return pls_integer; 
    function  ds_findmax(data clob) return pls_integer; 
     pragma restrict_references(ds_findmin, WNDS, WNPS); 
     pragma restrict_references(ds_findmax, WNDS, WNPS); 
end;

For the DataStream type and type body definitions, see Chapter 2, "Roadmap to Building a Data Cartridge".

For more information about PL/SQL packages, see the chapter about using procedures and packages in the Oracle9i Supplied PL/SQL Packages and Types Reference.

Pragma RESTRICT_REFERENCES

To execute a SQL statement that calls a member function, Oracle must know the purity level of the function, that is, the extent to which the function is free of side effects. The term side effect, in this context, refers to accessing database tables, package variables, and so forth for reading or writing. It is important to control side effects because they can prevent the proper parallelization of a query, produce order-dependent (and therefore indeterminate) results, or require impermissible actions such as the maintenance of package state across user sessions.

A member function called from a SQL statement can be restricted so that it cannot:

Insert into, update, or delete database tables
Be executed remotely or in parallel if it reads or writes the values of packaged variables
Write the values of packaged variables unless it is called from a SELECT, VALUES, or SET clause
Call another method or subprogram that violates any of these rules
Reference a view that violates any of these rules

For more information about the rules governing purity levels and side effects, see the PL/SQL User's Guide and Reference.

You use the pragma (compiler directive) RESTRICT_REFERENCES to enforce these rules. For example, the purity level of the DataStreamMax method of type DataStream is asserted to be write no database state (WNDS) and write no package state (WNPS) in the following way:

CREATE TYPE DataStream AS OBJECT (
         ....
PRAGMA RESTRICT_REFERENCES (DataStreamMax, WNDS, WNPS)
         ... );

Member methods that call external procedures cannot do so directly but must route the calls through a package. The reason is that currently the arguments to external procedures cannot be object types. A member function automatically gets a SELF reference (a reference to that specific instance of the object type) as its first argument. Therefore, member methods in objects types cannot call out directly to external procedures.

Collecting all external calls into a package makes for a better design. The purity level of the package must also be asserted. Therefore, when the package named DS_Package is declared and all external procedure calls from type DataStream are routed through this package, the purity level of the package is also declared, as follows:

CREATE OR REPLACE PACKAGE DS_Package AS
   ... 
PRAGMA RESTRICT_REFERENCES (ds_findmin, WNDS, WNPS)
   ...
end;

In addition to WNDS and WNPS, it is possible to specify two other constraints: read no database state (RNDS) and read no package state (RNPS). These two constraints are normally useful if you have parallel queries.

Each constraint is independent of the others and does not imply another. Choose the set of constraints based on application-specific requirements. For more information about controlling side effects using the RESTRICT_REFERENCES pragma, see the Oracle9i Application Developer's Guide - Fundamentals.

You can also specify the keyword DEFAULT instead of a method or procedure name, in which case the pragma applies to all member functions of the type (or procedures of the package). For example:

PRAGMA RESTRICT_REFERENCES (DEFAULT, WNDS, WNPS)

Privileges Required to Create Procedures and Functions

To create a standalone procedure or function, or package specification or body, you must have the CREATE PROCEDURE system privilege to create a procedure or package in your schema, or the CREATE ANY PROCEDURE system privilege to create a procedure or package in another user's schema.

For the compilation of the procedure or package, the owner of the procedure or package must have been explicitly granted the necessary object privileges for all objects referenced within the body of the code. The owner cannot have obtained required privileges through roles.

For more information about privilege requirements for creating procedures and functions, see the chapter about using procedures and packages in the Oracle9i Application Developer's Guide - Fundamentals.

Debugging PL/SQL Code

One of the simplest ways to debug PL/SQL code is to try each method, block, or statement interactively using SQL*Plus, and fix any problems before proceeding to the next statement. If you need more information on an error message, enter the statement SHOW ERRORS. Also consider displaying statements for runtime debugging, such as those of the general form:

Location in module: <location>
Parameter name: <name>
Parameter value: <value>

You can debug stored procedures and packages using the DBMS_OUTPUT package. You insert PUT and PUTLINE statements in your code to output the value of variables and expressions to your terminal. The DBMS_OUTPUT package is described in the Oracle9i Supplied PL/SQL Packages and Types Reference and the PL/SQL User's Guide and Reference.

To debug stored procedures and packages, though not object type methods at present, you can use Procedure Builder, which is a part of the Oracle Developer/2000 tool set. Procedure Builder lets you execute PL/SQL stored procedures and triggers in a controlled debugging environment, and you can set breakpoints, list the values of variables, and perform other debugging tasks. See the Oracle9i Java Stored Procedures Developer's Guide

A PL/SQL tracing tool provides more information about exception conditions in application code. You can use this tool to trace the execution of server-side PL/SQL statements. Object type methods cannot be traced directly, but you can trace any PL/SQL functions or procedures that a method calls. The tracing tool also provides information about exception conditions in the application code. The trace output is written to the Oracle server trace file.

Note:

Only the database administrator has access to this trace file. The tracing tool is described in the Oracle9i Application Developer's Guide - Fundamentals.

Notes for C and C++ Programmers

If you are a C or C++ programmer, several PL/SQL conventions and requirements may differ from your expectations. Note the following about PL/SQL:

= means equal (not assign).
:= means assign (as in Algol).
VARRAYs begin at index 1 (not 0).
Comments begin with two hyphens (--), not with // or /*.
The IF statement requires the THEN keyword.
The IF statement must be concluded with the END IF keyword (which comes after the ELSE clause, if there is one).
There is no PRINTF statement. The comparable feature is the DBMS_OUTPUT.PUT_LINE statement. In this statement, literal and variable text is separated using the double vertical bar (||).
A function must have a return value, and a procedure cannot have a return value.
If you call a function, it must be on the right side of an assignment operator.
Many PL/SQL keywords cannot be used as variable names.

See Also:
PL/SQL User's Guide and Reference.

Common Potential Errors

This section presents several kinds of errors you may make in creating a data cartridge.

Signature Mismatches

13/19    PLS-00538: subprogram or cursor '<name>' is declared in an object
         type specification and must be defined in the object type body
15/19    PLS-00539: subprogram '<name>' is declared in an object type body
         and must be defined in the object type specification

If you see either or both of these messages, you have made an error with the signature for a procedure or function. In other words, you have a mismatch between the function or procedure prototype that you entered in the object specification, and the definition in the object body.

Ensure that parameter orders, parameter spelling (including case), and function returns are identical. Use copy-and-paste to avoid errors in typing.

RPC Time Out

ORA-28576: lost RPC connection to external procedure agent
ORA-06512: at "<name>", line <number>
ORA-06512: at "<name>", line <number>
ORA-06512: at line 34

This error might occur after you exit the debugger for the DLL. Restart the program outside the debugger.

Package Corruption

ERROR at line 1:
ORA-04068: existing state of packages has been discarded
ORA-04063: package body "<name>" has errors
ORA-06508: PL/SQL: could not find program unit being called
ORA-06512: at "<name>", line <number>
ORA-06512: at line <number>

This error might occur if you are extending an existing data cartridge; it indicates that the package has been corrupted and must be recompiled.

Before you can perform the recompilation, you must delete all tables and object types that depend upon the package that you will be recompiling. To find the dependents on a Windows NT system, use the Oracle Administrator toolbar. Click the Schema button, log in as sys\change_on_install, and find packages and tables that you created. Drop these packages and tables by entering SQL statements of the following form into the SQL*Plus interface:

Drop type <type_name>;
Drop table <table_name> cascade constraints;

The recompilation can then be done using a SQL statement of the following form:

Alter type <type_name> compile body;
or
Alter type <type_name> compile specification;

5 Methods: Using PL/SQL