4 Implementing Data Cartridges in PL/SQL

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.

Map methods, which govern comparisons between object types, are discussed in the previous sections.

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

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);

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;

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;

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

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

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

salary := get_emp_sal(SELF => employee);

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;
);

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;

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, because 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 Oracle Database 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 Oracle Database 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 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.

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.

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

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

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>

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

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