You can use PL/SQL to implement data cartridge methods. Methods are procedures and functions that define the operations permitted on data defined using the data cartridge.
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.
Consider simple examples for implementing a method, invoking a method, and referencing an attribute in a method.
4.1.1 Implementing Methods
Example 4-1demonstrates the definition of an object type
The definition in Example 4-2 defines the function
gcd, which is used in the definition of the
normalize method in the
The statements in Example 4-3 implement the methods
plus for the object type
18.104.22.168 Defining an Object Type
Example 4-1 Defining an Object 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);
22.214.171.124 Defining a “Greatest Common Divisor” Function
Example 4-2 Defining a "Greatest Common Divisor" Function
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 normalizes (simplifies) 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;
126.96.36.199 Implementing Methods for an Object Type
Example 4-3 Implementing Methods for an Object 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;
4.1.2 Invoking Methods
To invoke a method, use the syntax in Example 4-4.
In SQL statements only, you can use the syntax in Example 4-5.
Example 4-6 shows how to invoke a method named
get_emp_sal in PL/SQL.
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.
In Example 4-7,
employee is the name of the object on whose behalf the
get_emp_sal() method is invoked.
188.8.131.52 General Syntax for Invoking Methods
Example 4-4 Invoking Methods; General Syntax
184.108.40.206 SQL Syntax for Invoking Methods
Example 4-5 Invoking Methods; SQL Syntax
220.127.116.11 PL/SQL Syntax for Invoking Methods
Example 4-6 Invoking Methods; PL/SQL Syntax
DECLARE employee employee_type; salary number; ... BEGIN salary := employee.get_emp_sal(); ... END;
4.1.3 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 := 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.
4.2 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
ERRORS. Also. consider displaying statements for run-time debugging. You can debug stored procedures and packages using the
DBMS_OUTPUT package, by inserting
PUTLINE statements into the code to output the values of variables and expressions to your terminal, as demonstrated in Example 4-9.
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.
The Oracle Database SQL Tuning Guide describes the tracing tool
The Oracle Database PL/SQL Packages and Types Reference and the Oracle Database PL/SQL Language Reference describe the
Example 4-9 Outputing Variable Values to the Terminal, for Debugging
Location in module: location Parameter name: name Parameter value: value
4.2.1 Notes for C and C++ Developers of Data Cartridges
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).
VARRAYsbegin at index
Comments begin with two hyphens (--), not with
IFstatement requires the
IFstatement must be concluded with the
IFkeyword (which comes after the
ELSEclause, if there is one).
There is no
PRINTFstatement. The comparable feature is the
PUT_LINEstatement. 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.
4.2.2 Common Potential Errors
Several kinds of errors that may occur while creating a data cartridge.
18.104.22.168 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.
22.214.171.124 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.
126.96.36.199 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 are 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 in the SQL*Plus interface, as shown in Example 4-10:
Example 4-10 Dropping Packages and Tables
Drop type type_name; Drop table table_name cascade constraints;
The recompilation can then be done using the SQL statements in Example 4-11:
Example 4-11 Recompiling Packages
Alter type type_name compile body; Alter type type_name compile specification;