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Oracle® Database Data Cartridge Developer's Guide
11g Release 1 (11.1)

Part Number B28425-01
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4 Implementing Data Cartridges in 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.

This chapter contains these topics:


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.

The following sections show simple examples of implementing a method, invoking a method, and referencing an attribute in a method.

See Also:

Implementing Methods

To implement a method, create the PL/SQL code and specify it within a CREATE TYPE BODY statement. If an object type has no methods, no CREATE TYPE BODY statement for that object type is required.

Example 4-1demonstrates the definition of an object type rational_type:

Example 4-1 How to Define an Object Type

( numerator INTEGER,
  denominator INTEGER,
  MEMBER FUNCTION plus (x rational_type)
       RETURN rational_type);

The definition in Example 4-2 defines the function gcd, which is used in the definition of the normalize method in the CREATE TYPE BODY statement later in this section.

Example 4-2 How to Define a "Greatest Common Divisor" Function

-- 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;
   IF (y <= x) AND (x MOD y = 0) THEN
      ans := y;
   ELSIF x < y THEN 
      ans := gcd(y, x);  -- Recursive call
      ans := gcd(y, x MOD y);  -- Recursive call
   END IF;
   RETURN ans;

The statements in Example 4-3 implement the methods rat_to_real, normalize, and plus for the object type rational_type.

Example 4-3 How to Implement Methods for an Object Type

CREATE TYPE BODY rational_type
   -- The rat-to-real function converts a rational number to 
   -- a real number. For example, 6/8 = 0.75
      RETURN numerator/denominator;

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

   -- 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
      r = rational_type(numerator*x.demonimator +
                 -- Example adding 1/2 to 3/4:
                 -- (3*2 + 1*4) / (4*2)
      -- Now normalize (simplify). Here, 10/8 = 5/4
      RETURN r;

Invoking Methods

To invoke a method, use the syntax in Example 4-4:

Example 4-4 How to Invoke Methods; General Syntax


In SQL statements only, you can use the syntax in Example 4-5:

Example 4-5 How to Invoke Methods; SQL Syntax


Example 4-6 shows how to invoke a method named get_emp_sal in PL/SQL:

Example 4-6 How to Invoke Methods; General Syntax

   employee employee_type;
   salary number;
   salary := employee.get_emp_sal();

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, Example 4-7 performs the same action as the salary := employee.get_emp_sal(); line in Example 4-6:

Example 4-7 How to use the SELF Build-In Paramenter

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

Because member methods can reference the attributes and member methods of the same object type without using a qualifier, a built-in reference, SELF, always identifies the object on whose behalf the method is invoked.

Consider Example 4-8, where two statements set the value of variable var1 to 42:

Example 4-8 How to Set Variable Values

   var1 INTEGER,
   MEMBER PROCEDURE set_var1);
      var1 := 42;
      SELF.var1 := 42;
   END set_var1;

The statements var1 := 42 and SELF.var1 := 42 have the same effect. 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.

Example 4-9 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.

Example 4-9 How to Create a Package Specification

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); 

See Also:


To execute a SQL statement that calls a member function, Oracle must know the purity level of the function, or the extent to which the function is free of side effects. The term side effect, 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:

You must use the pragma RESTRICT_REFERENCES, a compiler directive, to enforce these rules. In Example 4-10, 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).

Example 4-10 How to Assert the Purity Level of a Type

         ... );

Member methods that call external procedures cannot do so directly but must route the calls through a package, because the arguments to external procedures cannot be object types. A member function automatically gets a SELF reference 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 demonstrated in Example 4-11:

Example 4-11 How to Assert the Purity Level of a Package


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.

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, as demonstrated Example 4-12.

Example 4-12 How to Assert a Default Purity Level for All Type Methods and Package Procedures


See Also:

Privileges Required to Create Procedures and Functions

To create a standalone procedure or function, or a package specification or a 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 Oracle Database Advanced Application Developer's Guide.

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. You can debug stored procedures and packages using the DBMS_OUTPUT package, by inserting PUT and PUTLINE statements into the code to output the values of variables and expressions to your terminal, as demonstrated inExample 4-13.

Example 4-13 How to Output Variable Values to the Terminal, for Debugging

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

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 that only the database administrator has access to the file.

See Also:

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

  • = 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.

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;
Alter type <type_name> compile specification;